ARNOLD'S GEOLOGICAL SERIES
General Editor; DR. J. E.' MARK, F.R.S.
THE GEOLOGY OF BUILDING STONES
BY
J. ALLEN HOWE, B.Sc., F.G.S.
MEMBER OF THE COMMITTEE ON BUILDING STONES AND MORTAR, INTERNATIONAL ASSOCIATION FOR THE TESTING OF MATERIALS
LONDON EDWARD ARNOLD
1910
{All rights reserved}
EDITOR'S PREFACE
THIS is the fourth volume of a series of works treating of economic geology, the previous books having been devoted to ' The Geology of Coal and Coal-Mining,' ' The Geology of Ore Deposits,' and ' The Geology of Water- Supply.'
A work on * The Geology of Building Stones,' dealing with the great advance made in their study both at home and abroad of recent years, was much needed, and the author of the present work, under whose direction as Curator of the Museum of Practical Geology in London the collection of building stones in that museum assumed its present shape, is peculiarly qualified
for the task of writing the work.
J. E. MARK.
111
AUTHOR'S PREFACE
IN the following pages an attempt has been made to gather together some of the facts about the Geology of Building Stones, mainly with a view to the requirements of students of architecture.
Special attention has been given throughout to the materials found in the British Isles; but it has been thought desirable to add brief references to some of the stones of other countries.
Naturally, in dealing with a subject covering so wide a field, selection and much condensation have been inevitable. Limitations of space have prevented the con- sideration of Ornamental Stones, and the introduction of references to the very numerous sources of information.
The writer's thanks are due to Professor T. Hudson Beare and Mr. W. R. Baldwin- Wiseman for permission to quote certain tables from their exceedingly valuable papers, and to the Councils of the Institution of Civil Engineers and of the Surveyors' Institution for kindly giving their consent.
Thanks are due also to Professor Buckley for permission to reproduce Fig. 28 ; to the Controller, His Majesty's Stationery Office, for Figs, u, 19, 20 and 22; to the United States Geological Survey for Fig. 16, from
'
vi AUTHOR'S PREFACE
Dr. Nelson Dale's important Bulletins; to Mr. T. C. Hall for assistance with some of the photographic plates ; and especially to Professor J. Hirschwald, of Charlotten- burg, for generously allowing the reproduction of several figures and tables from his monumental work, ' Die Prii- fung der natiirlichen Bausteine.'
In common with all who are interested in the subject, the writer is indebted to Professor McKenny Hughes and Mr. John Watson for the fine collection of building stones now forming at Cambridge.
Finally, he wishes to record his indebtedness to his colleagues, and the many architects, quarry-owners, and merchants who have assisted him with information and advice, and particularly to Dr. J. S. Flett, who has most kindly looked through the proofs, and to Mr. H. B. Wood- ward and Mr. W. Brindley, whose stores of knowledge have always been freely opened and freely used.
J. A. H.
JERMYX STREET, S.W., November, 1910.
CONTENTS
CHAPTER PAGE
I. INTRODUCTORY I
TABLE OF STRATA - u
II. MINERALS - - 13
III. IGNEOUS ROCKS — GRANITE - - 35
IV. IGNEOUS ROCKS OTHER THAN GRANITE 82 V. SANDSTONES AND GRITS 112
VI. LIMESTONES - 173
VII. SLATES AND OTHER FISSILE ROCKS 273
VIII. MISCELLANEOUS BUILDING STONES - 327
IX. THE DECAY OF BUILDING STONE - 333
X. THE TESTING OF BUILDING STONES - 362
MOHS'S SCALE OF HARDNESS - 41 1
APPENDICES :
A. GRANITE QUARRIES - - 412
B. CLASSIFIED LIST OF LARGER SANDSTONE
QUARRIES - 4l6
C. CLASSIFIED LIST OF LARGER LIMESTONE
QUARRIES - 424
D. LIST OF CHIEF SLATE QUARRIES 43°
E. SOME USEFUL BOOKS 433
INDEX 436 vii
LIST OF PLATES
PLATE FACING PAGE
I. MINERALS - 14
II. IGNEOUS ROCKS - 48
III. PHOTOMICROGRAPHS OF SANDSTONES - 122
IV. PHOTOMICROGRAPHS OF LIMESTONE — (A) OOLITE,
KETTON STONE J (B) CRINOIDAL AND SHELLY, FROM
HOPTON WOOD - - 192
V. PHOTOMICROGRAPH OF LIMESTONE, HAM HILL SHELLY 206 VI. PHOTOMICROGRAPH OF LIMESTONE, BATH OOLITE, BOX
GROUND 234
VII. PHOTOMICROGRAPHS OF VARIOUS LIMESTONES - 262
VIII. WEATHERING OF PORTLAND STONE - - 350
LIST OF MAPS
MAT PAGE
I. IGNEOUS ROCKS OF ENGLAND AND WALES - 71 II. TRIASSIC, DEVONIAN ORDOVICIAN AND CAMBRIAN
SYSTEMS OF ENGLAND AND WALES - 129
III. GEOLOGICAL MAP OF IRELAND - 134
IV. GEOLOGICAL MAP OF SCOTLAND 137 V. CRETACEOUS AND CARBONIFEROUS SYSTEMS OF ENGLAND
AND WALES 142 VI. JURASSIC, PERMIAN, DEVONIAN AND SILURIAN SYSTEMS
OF ENGLAND AND WALES - 189
VII. SLATE DISTRICTS OF ENGLAND AND WALES - - 292
THE GEOLOGY OF BUILDING STONES
CHAPTER I INTRODUCTORY
THERE is no help : sooner or later, in the course of prac- tice, the architect or engineer will have the need of some geological knowledge forced upon him. Force, indeed, is seldom required ; for brought as he is into direct contact with geological problems, and having by the nature of his training developed an inquiring mind, the architect or engineer has often become a willing student of the science.
The influence of the geological structure of a country upon the character of its inhabitants, upon their indus- tries, and upon the location of their towns and villages, hardly needs to be elaborated. But more definitely the influence of geological environment makes itself felt in the architectural ideals of a people and their mode of expression. Here I may quote from the ' Stones of Venice,' not because Ruskin's geology is sound — it is bad —but because of the truth underlying his statement. He says, speaking of monolithic shafts : ' It is clearly necessary that shafts of this kind (we will call them, for convenience, block shafts) should be composed of stone not liable to flaws and fissures; and, therefore
2 GEOLOGY OF BUILDING STONES
that we must no longer continue our arrangement as if it Were always possible to do what is to be done in the best way ; for the style of a national architecture may evidently depend, in a great measure, upon the nature of the rocks of the country.
' Our own English rocks, which supply excellent build- ing stone from their thin and easily divisible beds, are for the most part entirely incapable of being worked into shafts of any size, except only in the granites and whin- stones, whose hardness renders them intractable for ordinary purposes; — and English architecture therefore supplies no instances of the block shaft applied on an extensive scale ; while the facility of obtaining large masses of marble has in Greece and Italy been partly the cause of the adoption of certain noble types of archi- tectural form peculiar to those countries, or, when occur- ring elsewhere, derived from them. . . . The shaft built of many pieces is probably derived from, and imitative of, the shaft hewn from few or from one.
' If, therefore, you take a good geological map of Europe, and lay your ringer upon spots where volcanic influences supply either travertine or marble in accessible and avail- able masses, you will probably mark the points where the types of the first school have originated and developed. If, in the next place, you will mark the district where broken and ragged basalt and whinstone, or slaty sand- stone, supply materials on easier terms indeed, but frag- mentary and unmanageable, you will probably distinguish some of the birthplaces of the derivative and less graceful school. You will, in the first case, lay your finger on Paestum, Agrigentum, and Athens ; in the second, on Durham and Lindisfarne.'
This, in a sense, is true, but other and more complex influences have usually been at work ; yet in spite of many exceptions, abundant evidence of the reality of the
INTRODUCTORY 3
interaction between geology and architectural forms may be found without even crossing the Channel.
It is surprising to see how sharply and clearly the older domestic and often the ecclesiastical architecture of this country corresponds with the local geological formations.
Look for example, at the Chalk country of the south- east of England ; here is the home of flint decoration, best developed in the eastern and southern counties, where this material is most readily obtained. But even where the Chalk has disappeared, and the residual flint, in the later gravels, is all that remains of it, these remnants are still employed in a similar way, as may be seen on the fringes of the Haldon Hills in Devonshire. In the same county, too, may be seen those charming structures the * cob-walled ' farmsteads, only too rapidly passing away. These are built of adobe, or earth moulded with stones and straw or hair. They are found also in Cornwall and Somerset, always on the tracts where the decomposed Devonian or Carboniferous shales and slates give rise to surface deposits of stiff greyish clay ; or on the tracts of New Red marl, from which cob walls of bright red hue were fashioned. How closely the material follows the geological formation of the ground is shown in a striking manner at Hatherleigh, where on the narrow strip of New Red marl, the extreme limit of a long outcrop stretching out from the east, we find the bright red houses on a belt of rock only a few hundred yards wide, surrounded on either side by cottages of the grey and buff of the Culm.
In a similar way the picturesque half-timbered houses of Cheshire, Worcestershire, Salop, and Gloucestershire, and some of the adjoining counties, are found on the New Red marls or the Old Red marls, which yield material of the same kind, in earlier times for cob walls, and later for bricks.
The limestone and grit districts of the Carboniferous
4 GEOLOGY OF BUILDING STONES
are equally well distinguished by their own type of build- ing ; so is the slate country of Wales, Cornwall, and the Lakes, and the granite country of the West of England and parts of Scotland ; and this is true not only as regards general appearances, but in many of the details of construction.
The same close relationship between the buildings and the geological characters is to be seen with equal clearness during a rapid traverse of Europe by rail, no matter what direction be taken.
There are again many minor points affecting the security of dwellings and other structures upon which the knowledge of the geologist is necessary to the archi- tect, but which space forbids us to consider here.
For example, we find the neglect of ' hill creep ' resulting in yawning gaps in boundary walls, leaning gateposts, and cracking houses, when a little more depth to the foundation or the addition of on occasional buttress would have insured safety. This steady, slow, but unceasing, descent of the upper layers of all soft rocks, such as shales and clays, towards the bottom of the slope should always be counteracted by greater depth and solidity in the foundations.
Besides the slow downward creep of the surface layers, there is often a tendency, in shaly rocks and clays or among hard rocks resting on clays or shales and dipping towards the valley, for larger masses to move forward in the form of slips.
In the following chapters an attempt has been made to place before the student some of the salient geological characters of building stones ; their mode of occurrence and physical and chemical properties.
Here it is necessary that the student should be warned :
INTRODUCTORY 5
These pages do not undertake to give any adequate ex- planation of the elementary principles of geology. Thus, although there is a short section on the minerals in building materials, the student must turn to a treatise on Mineralogy if he is to grasp the meaning and technology of crystalline form and habit, and the significance of the association of one mineral with another. Again, although there is a brief introduction to the igneous rocks, he must study some work on Petrology if he is to follow satis- factorily the descriptions of the rocks in thin slices under the microscope and in hand specimens, and he must supplement this by referring to some book on General Geology in order to understand the various relationships of these rock masses in the field and their mode of formation.
Similarly, in the case of the stratified sedimentary rocks, the student may learn from the general textbooks how they are formed and solidified, how they and the igneous rocks are bent, faulted, or broken ; how they may be altered or metamorphosed ; how they are worn and carved by the agencies of weathering into hills and valleys, plains and escarpments.
It is not, however, from books that geology is to be learned ; nothing but personal acquaintance with the things will put the facts and the arguments founded upon them in their true perspective ; and this kind of knowledge it is not so difficult to acquire, though it takes time.
It is easy enough to describe a sandstone so that it will be distinguishable from a limestone, and the two stones could be recognized from the description by one who had never seen them before ; but to describe twenty sand- stones in a way that would lead to their immediate recognition is a task demanding much time and more skill than the writer possesses. Yet how to recognize a stone at sight from amongst a host of others of the same kind is
6 GEOLOGY OF BUILDING STONES
just one of the things that the architect wants to know. The books will not tell him ; but once he has seen the stone long enough to fix its appearance in his mind, and has noted the ' feel ' of it, it is unlikely altogether to escape him again.
Having become accustomed to the stone, it may then be profitable to look up the microscopic structure, the chemical composition, the physical properties, etc., as set down in the books. In this way the student's own experience and the observations of others may be welded into a stock of sound information.
' Learn to know about your materials,' said Mr. R. W. Schultz recently in addressing a society of architects ; Vgo to a quarry to see stone actually quarried, go to a mason's yard to see it being dressed, try to do some yourself if he will let you — see it built into position, look at older buildings where the same stone has been used, see how it weathers under different conditions, whether the mouldings decay rapidly (if so, it should not be moulded), what form of surface wears best, and you will find lots of wrong uses and some right ones ; but before you have done you will know, or ought to know, what to avoid doing. Learn something in a similar way about limes, sands, wood, plaster, paint — in short, everything that goes to make a building."
More than one President of the Royal Institute of British Architects, and other well-known authorities in the profession, have repeatedly given similar advice.
By all means let this advice be carried out on every opportunity, but even then the busy student will have small chance of seeing more than a few of the better, known quarries during the period when there are so many other urgent calls on his attention. It is hoped that the brief geological and topographical notes on the building stones which follow may be of assistance to the student in
INTRODUCTORY 7
supplementing the knowledge he is able to gain at first hand.
Now for a word of warning to the seasoned practitioner who may be tempted to search these pages for some information respecting an unfamiliar stone. It is greatly to be feared he will often be disappointed. He would naturally like to find clear descriptions of all the kinds of stone he is likely to meet in the pursuance of his art ; he might like to know where they are to be found, in what sizes they may be obtained, how much they cost, for what purposes they are suitable or unsuitable ; he might like to know their chemical and physical properties, and their behaviour when subjected to all kinds of tests; and, finally, he would like to know where examples of the stone may be seen in old buildings or in new, in order that he might observe for himself how it had withstood the supreme test of time and exposure.
In no sense does the present volume attempt to fulfil these ideal requirements ; if it makes clearer the geological characters of some of the stones, their relationships and distribution, and exhibits the value of some of their chemical and physical properties in true proportions, that is as much as can be hoped.
The treatment of each group is in the first place geological, and in the second place topographical, and many stones of minor importance have been mentioned, because not infrequently a client insists upon having a stone used from his own estate, and a knowledge of what is likely or unlikely to be found on it may be useful.
In this connection it may not be out of place to mention that geological maps on the scale of i inch to the mile are available for practically the whole of Great. Britain and Ireland, and that maps on the larger scale of 6 inches to the mile have been prepared to show the geology of many large areas. These maps and their. accompanying
8 GEOLOGY OF BUILDING STONES
descriptive memoirs (which are full of local detail) may be consulted by anyone in the library of the Geological Survey and Museum, Jermyn Street, London.
A fairly complete series of British and some foreign building stones is exhibited in the same institution. Some colonial building materials are shown at the Imperial Institute, and a fine general collection is housed in the Geological Museum at Cambridge.
The Testing of Building Stones, a subject of great complexity and difficulty, is dealt with — so far as space permits — in Chapter X. ; how far the conclusions there arrived at will meet with acceptance will doubtless depend upon individual experience and inclination. Although many of the tests there indicated are confessedly both reasonable and useful, yet it must be admitted that so far in actual practice — largely, perhaps, because really com- parable tests are as yet so few — they are rarely, very rarely, made use of by the architect, the builder, or the engineer. In short, the common sentiments in this con- nection are akin to those of Omar's Pot. ' Said one, They talk of some strict Testing of us — Pish !'
Take, for example, the simplest and most commonly applied test, the chemical analysis ; it reveals, amongst other things, the presence of lime ; and how often has it not been reiterated by the learned that ' no stone con- taining lime should be used in towns.' Even if we limit this prohibition to stones containing carbonate of lime, how does theory square with practice ? We find fully three-fourths of the large buildings erected in London during the past few years are either wholly constructed or faced with the deleterious substance, or carry it in the prominent dressings. It will be sufficient to mention the Government buildings in Whitehall and Westminster, the Piccadilly Hotel, the Victoria and Albert Museum, to see how our architects are influenced by this test. Even
INTRODUCTORY 9
the national monument to Queen Victoria, precisely the type of structure which all men wish to be permanent, is fashioned out of carbonate of lime.
It is true there are limestones and limestones ; but it is not comparative and so-called scientific testing which has hitherto determined the use of one or another ; it is a question of custom, appearance, and cost. We know that as towns are to-day, the limestones will certainly, and in some cases rapidly, decay ; and it does not take long for all ornament to be obliterated, yet we continue to cut into the limestone elaborate designs which cannot last in perfection for ten years. We know, further, that this will continue to be the fate of limestone buildings until we have learned to consume our own smoke.
The choice of a building stone is often no easy matter. The architect may wish to use a granite for one part of the building, as an emblem of strength, and Portland stone for another, that he may have the beautiful effect of its slow and even decay ; but the question of cost may prohibit the use of the first, and a north-east aspect may render unsuccessful the employment of the second.
The limitations imposed by situation must always exert a powerful influence on the choice of stone ; much is possible in the country that is unattainable in our smoky towns, both as regards colour and texture. The retention of the stone's natural colour and the mellowing effects of age may be reckoned on and allowed for with far more freedom in country buildings — we may even anticipate with satisfaction
' the lichen'd wall Whose lizard hues are painted by slow Time ;'
but the appearance of these delights on a town building would only call forth the steam-brush or the drag.
Cost is the great arbiter oftener than not; fitness and beauty have perforce to give way before £ s. d.
io GEOLOGY OF BUILDING STONES
Fear, the fear of risk, prevents the use of many good stones with which the architect is unfamiliar, and it is here that the absence of reliable authoritative tests is felt, and ready information as to the resources of quarries is needed.
There is no single stone which is perfectly appropriate for all purposes, but there is abundant variety to choose from among the granites, limestones, and sandstones, sufficient to meet every requirement as to strength, colour, and texture : it is the duty of the student to familiarize himself with as many of these as possible, and if he makes his choice by the application of sound principles and common-sense, and with personal care sees that the quality is upheld, there is little danger of subsequent disappointment.
INTRODUCTORY
ii
Pleistocene
Pleiocene
(Miocene) Oligocene
Eocene
Cretaceous
Jurassic
TABLE OF STRATA.
Old river gravels and loams, estuarine beds, and raised beaches
Gravel for ballast, road metal, concrete ; build- ing sand, brick earth, silt for cement
Glacial deposits yielding building sand, brick earth, gravel, road metal, and occasionally rough walling stone
Cromer Forest
Beds Norwich and Red
Crags Coralline Crag
Hamstead, B e m b r i d g e , Osborne, and Headon Beds
Barton, Brackle- sham, and Bag- shot Beds
London Clay
Oldhaven and Blackheath Beds
Woolwich and Reading Beds
Thanet Sand
Chalk
Upper Green- sandandGault
Lower Green- sand
Wealden Beds Purbeckian
Portlandian Kimeridgian Corallian Qxfordian
Building sand, brick earth, gravel, road metal
Building stone, limet, cement, brick earth
Glass sand, road metal, setts, building stone, cement stone
Brick earthi Flint gravel
Brick and tile clay, building
stone, setts Moulding sand
Lime, cement, whiting, build- ing stone, flint Road metal, building stone,
hearthstone, brick clay Building stone, road metal,
glass sand, building sand,
brick clay Brick clay, sand, building stone,
Sussex marble Purbeck marble, Swan age
stone, lime, gypsum, road
metal, tilestones Portland building stone, lime,
sand, sandstone Brick and tile clay, cement
stone Building stone, lime, road
metal, brick clay Brick and tile clay
12
GEOLOGY OF BUILDING STONES
TABLE OF STRATA — Continued.
|
0 O c/3 o ^ |
'Cornbrash Forest Marble and Bradford Clay |
Lime and local building stone Lime and local building stone |
|
13$ |
Great Oolite |
Building stones, stone tiles, |
|
u |
road metal |
|
|
o |
.Fuller's Earth |
— |
|
Inferior Oolite |
Building stone, lime, stone |
|
|
Series |
tiles, sand |
|
|
Lias |
Building and paving stone, |
|
|
brick clay, lime, cement |
||
|
Trias |
Rhaetic |
Building stone |
|
Keuper |
Building stone, brick and tile |
|
|
clay, gypsum |
||
|
Bunter |
Building stone, sand, road |
|
|
metal, pebble gravel |
||
|
Permian |
Red Marls, Sand- |
Building stone, brick and tile |
|
stone and Mag- |
clay |
|
|
nesian Lime- |
||
|
stone |
||
|
Carboniferous |
Coal - measures |
Building and paving stone, |
|
(and Culm |
stone tile-, brick and tile |
|
|
rocks) |
clay, fire clay |
|
|
Millstone Grit |
Building and paving stone |
|
|
Lower Carbon- |
Building stone, marble, lime, |
|
|
iferous Rocks |
road metal |
|
|
(Old Red |
Red Marls and |
Building and paving stone, |
|
•! Sandstone |
Sandstones |
brick clay |
|
v Devonian |
Limestone, |
Building stone, lime, marble, |
|
Shales, and |
road stone, slate |
|
|
Sandstone |
||
|
Silurian |
Ludlow Beds |
Building stone, lime, slates |
|
Wenlock Beds |
Lime, paving stones |
|
|
Llandovery Beds |
Building and paving stone, |
|
|
lime, slate |
||
|
Ordovician |
Bala and Car- |
Lime, jasper |
|
adoc Beds |
||
|
Llandeilo Beds |
Slates |
|
|
Arenig Beds |
Slates |
|
|
Cambrian |
Tremadoc Beds |
Slates |
|
Lingula Flags |
Building stone, paving stone |
|
|
Menevian Beds |
Slates, paving stone |
|
|
Harlech Beds |
Building stone, slate, road metal |
Pre-Cambrian Road metal and local building
stone Igneous rocks occur on severa.1 horizons,
CHAPTER II MINERALS
THE more we know of the true nature of ' minerals,' the greater our difficulty in denning them in a few plain words. It may be said that they are the materials which constitute the solid crust of the earth ; but since some of these solid materials are liquefied occasionally, and even converted to a gaseous condition by local heating, in regions of volcanic activity, or are normally liquid and only solidified by local cooling, as in the case of water in the polar regions, it would be difficult to exclude from the term ' mineral ' those portions of the crust which commonly exist in the state of liquids or gases. Even from among the building materials water cannot be altogether excluded, for the Eskimos find in it a con- venient and durable * freestone.' Interesting relations may also subsist between organic bodies and minerals ; for instance, the wooden piles which form the foundations of the houses in parts of Nicaragua are found after about fifty years to be converted in their lower parts into hard siliceous stone.
When, however, the term ' mineral ' is employed in its everyday sense, certain definite kinds of mineral matter are connoted.
The different kinds of mineral substances can be recog- nized and distinguished by the possession of certain definite characters which are the peculiar property of each kind.
13
14 GEOLOGY OF BUILDING STONES
Two distinct properties must be possessed by a mineral substance before it can be recognized as a separate kind ; these are its chemical composition and the expression of its peculiar molecular construction. Two mineral sub- stances may have precisely the same chemical composition, but if the form-type differs — due to differences in molecular arrangement — they will not be regarded as one mineral kind. Thus, calcite (CaCO3) and aragonite (CaCO3) have the same composition, but their molecular construction, as expressed by their crystalline form, is different. With the peculiarities of molecular arrangement are associated several properties which serve to distinguish the mineral and give it characteristic optical, electrical, thermal, and elastic features.
One kind of mineral may occur in many minor variations of form, colour, and habit, without losing its essential characters. Minerals may occur with regular external crystalline form as crystals — (Plate I.) ; or they may have easily recognizable crystalline structure without the external regularity of form (for example, most of the quartz grains in granite) — (Plate II.) ; or they may be amorphous, without crystalline structure.
Minerals are formed and grow from aqueous solutions, cold or warm, or from molten liquid rock magmas. Some mineral substances have been formed through the media- tion of organisms — e.g. from plants, coal and diatomaceous earth ; from animals, many limestones. A few minerals are deposited by sublimation from a gaseous condition, but they are not of the kind that should appear in building materials.
Through the action of the various agencies in their environment, minerals of one kind are continually being modified or broken up and re-formed as minerals of other kinds.
PLATE I
MINERALS.
Felspar : a, c, and d, crystals (d is a twinned form) ; l>, a Cleavage face, red felspar
f, felspar (light) intergrown with quartz (dark) in ' graphic ' granite. Quartz : e, crystalline mass ; g, transparent crystal. Pyroxene : h, Augite.
Amphibole : i, hornblende ;/, actinolite in quartz schist (mica in upper right corner) Mica : k, portion of large crystal. Calcite : /, dogtooth spar ; m, cleavage rhomb in yellow calcite ; », nail-head spar.
MINERALS 15
Rocks are built up of minerals associated in an infinite variety of ways, and the following brief notes upon some of the more important rock-forming minerals are inserted here for the assistance of the reader, who will find them referred to in some of the subsequent chapters.
To gain any knowledge whatever of the real character of these minerals, it is essential that the student should see and handle the minerals themselves. Their physical and chemical properties, and their mode of occurrence, will be found described in works on mineralogy (see p. 433), but for their appearance and characters when examined in thin slices under the microscope, books on petrology must be consulted.
The number of different kinds of mineral that go to build up the earth's crust is comparatively small, well under one thousand, and of these only a dozen or so are important rock-formers.
Quartz.— Oxide of Silicon (SiO2). H. = 7. Sp. gr. = 2*65. Crystallizes in the Hexagonal system. Insoluble in acids. Fuses at about 1600° C. ; vitreous lustre, conchoidal fracture.
This is the most widely spread and important of the minerals occurring in building stones. It occurs in grains as an essential constituent of all granites and felsites, and as a subordinate mineral in a host of other igneous rocks. It forms the great majority of sands, sandstones, and grits, in the state of grains derived from pre-existing igneous and sedimentary rocks. As a cementing material it is common in many sandstones and shales, and there are few limestones, clays, or slates, in which it does not appear.
In mineral veins which traverse rocks of all kinds, and in hollows and druses, it is frequently found with well- marked crystal faces. (Plate I. and Fig. i.)
The colour of quartz is subject to a wide range of varia-
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MINERALS 17
tion. Very commonly the grains are colourless and glasslike ; large crystals of this character are known as ' rock - crystal.' It may be 'smoky,' yellow, brown to black ; amethystine, violet or purple, reddish (inclusions of haematite) ; rose-coloured (titanium or manganese) ; milky or opaque, as in vein quartz. Most of these varieties may be observed in the quartz of granites. The opalescent appearance of some quartz is caused by a fibrous structure due to a mineral like asbestos which has been replaced by silica.
Granitic quartz frequently contains many sheets of minute vacuoles, which may contain bubbles of liquid (water or carbon dioxide) and gas (Fig. 2, d). Their presence may possibly exert an influence upon the cleav- ability of the stone and upon its behaviour when subjected to fire.
Although quartz has no tendency to cleave readily in any direction under normal conditions, if it is plunged into cold water suddenly after being heated it tends to fracture with a rough rhombohedral cleavage.
Silica plays an important part in many rocks in other forms than that of quartz.
Chalcedony is one of these varieties of natural silica; it is an apparently amorphous substance, having a waxlike lustre and even fracture. It is translucent; its specific gravity is 2'6, and its hardness is equal to that of quartz. Examined under the microscope, it is seen to be composed of minute crystalline fibres ; it is not really amorphous. It may be colourless, grey, white, yellow, brown, dusky blue, or black. It occurs in sedimentary rocks, sandstones, and limestones, as a cement, filling up interstices or ap- pearing in patches or replacing organic fragments. In igneous rocks it is found filling vesicles, lining geodes, and forming small veins. Banded varieties are called agates.
i8 GEOLOGY OF BUILDING STONES
Opal is a form of hydrated silica (SiO2.wH2O). H. = 6. Sp. gr. = 2'i. Unlike chalcedony, it is amorphous. The ordinary content of water ranges between 5 and 10 per cent. ; but some varieties have as much as 30 per cent, while others are almost anhydrous. It occurs in rocks in much the same way as chalcedony. It is found in a dull white condition, or with the lustre and play of colour of the well-known precious opal. Wood opal is a replacement of woody tissue by opal (see p. 13).
Jaspers are impure siliceous rocks composed of quartz in very minute particles mixed with argillaceous matter, and coloured by yellow, red, and dark brown hydroxides of iron. Jasper is opaque and somewhat dark in appear- ance ; its strong coloration and hardness have led to its frequent employment as a decorative stone. Most jaspers are due to the metamorphosis of argillaceous or calcareous rocks by aqueous solutions.
Next to quartz in importance, among the minerals of building materials, are the Felspars. These are silicates of alumina, alkalies, and lime ; they form a very clearly denned group, the members of which are closely related by their physical and chemical properties. According to their mode of crystallization, they fall into two groups : Orthoclase felspar (monoclinic) and plagioclase felspar (anorthic). An important character in these minerals is the presence of two well-marked directions of cleavage nearly at right angles.
Orthoclase is a potash felspar (KAl.Si8O8) ; it crystal- lizes in the monoclinic system. Its hardness is 6 ; it melts at 1150° C., and it is insoluble in acids. When it has had space to build perfect crystals, it assumes forms such as those indicated in Fig. i.
Of course, the perfect forms indicated in the figures are rarely seen in the broken or polished surfaces of the stones
MINERALS 19
in which orthoclase is found ; the crystals appear usually as stout prisms or broad plates (Plates I. and II.). Twin- ning is usually recognizable by the change in the amount of reflection from the adjacent portion of the crystal as the stone is turned about.
In appearance orthoclase is colourless and glassy in the varieties Sanidine and Adularia, but it is usually some- what turbid white, grey, yellowish or flesh-coloured, due partly to a cloudy kaolinized alteration of the crystal, stained more or less with oxides of iron. The milky appearance of the felspar in granite, etc., is partly due to the effect of the minute cleavage flakes in reflecting the light.
The pearly, pale bluish opalescence of some orthoclase is due to minute inclusions or decomposition, or to layers of different composition (see below) parallel to the ortho- pinakoid or orthodome.
Orthoclase is a very characteristic constituent of granites and syenites.
Among the plagioclastic felspars there are two which must receive prior consideration, because the researches of Tschermak and others have shown that many of the plagioclases are neither more nor less than mixtures or solid solutions of these two — Albite, soda felspar, and Anorthite, lime felspar — in varying proportions. These mixtures may be represented by the general formula
m Albite (Ab) + n Anorthite (An) or
wNaAl.Si3O8 + «CaAl2Si2O8.
The close relationship between the composition of the felspars and their physical properties may be seen by reference to Table I.
The chemical composition of the pure felspars is show in Table II.
Between the plagioclase felspars enumerated in the two
20
GEOLOGY OF BUILDING STONES
tables there are others of intermediate composition which do not receive distinctive names. The names Oligoclase, Andesine, etc., are retained for convenience, to denote fel- spars of a certain range of composition in a series of mixtures of the albite and anorthite molecules.
TABLE I.
|
Name of Felspar. |
Short Formula. |
Specific Gravity. |
SiO. |
Extinc- tion on Face C. |
Extinc- tion on Face B. |
Mean Refrac- tive Index. |
||
|
Orthoclase |
_ |
2^65 |
647 |
0° |
t- 5° |
•523 |
/ |
S <n •o |
|
Microcline |
|
2-56 |
64-6 |
+ 15° 30' |
+ 5° |
•523 |
>, |
1 |
|
Albite ... |
Ab |
2-624 |
68-7 |
+ 4° 30' |
+ 19° |
'534 |
C |
|
|
Oligoclase- |
15 |
* |
||||||
|
Albite |
AbgAnj |
2-645 |
64-9 |
+ 2° 45' |
+ 12° |
— |
J |
1 |
|
Oligoclase |
Ab-jAiij |
2-659 |
67-0 |
+ i°4' |
+ 4° 36' |
•542 |
.5 |
1 |
|
Andesine |
Ab2Anx |
2-694 |
60-2 |
- 5° 10' |
-16° |
•558 ! 3 |
bJO |
|
|
Labradorite |
AbtAn3 |
2-728 |
49'3 |
-17° 40'; -29° 28', -570 |
J |
|||
|
Bytownite |
AfyAiig |
2742 |
46-6 |
-27° 33' -33° 29'! |
^ o |
|||
|
Anorthite |
An |
2-758 |
43'2 |
-37° |
-36° |
1-582 |
•Svl |
TABLE II.
|
Felspar. |
Si02. |
A1203. |
K20. |
NaO. |
CaO. |
|
Orthoclase |
64-7 |
l8-4 |
16-9 |
_ |
|
|
Albite |
68-7 |
19*5 |
— |
n-8 |
— |
|
Oligoclase Labradorite .. |
62-0 60-2 |
24-0 25-2 |
|
8-7 7*9 |
5*3 67 |
|
Andesine |
5i7 |
30-9 |
— |
4.0 |
*3*4 |
|
Bytownite |
47'4 |
33-» |
— |
2-0 |
16-8 |
|
Anorthite |
43-12 |
36-7 |
~ |
20'I |
The fact that some of these plagioclase crystals are mixtures of the kind indicated cannot be observed except indirectly by chemical analysis or examination of the optical or other properties which accompany the changes in composition (see Table I.). In some crystals, however,
MINERALS 21
the mixture is revealed by the microscope, when minute inclusions of one felspar appear intergrown with another (see Perthite) ; and not infrequently a single crystal is shown, by its behaviour in polarized light, to be composed of a series of felspars passing gradually from a less siliceous variety at the centre to a more siliceous one at the periphery.
It has been pointed out that, since there is usually a little soda in orthoclase, presumably due to an ultra- microscopic intermixture of albite, it is quite possible that its monoclinic character may be apparent and not real — that it may, in fact, be anorthic with a pseudo-symmetry. There is, moreover, a distinctly anoithic potash felspar of the same composition as orthoclase ; this is Microcline, an important rock-forming mineral. Its outward form cannot be distinguished from that of orthoclase in the rough stone.
Another alkali felspar, Anorthoclase — (NaK)Al.Si3O8 — containing soda (and a little lime), shows a cross- hatching like microcline. It is exemplified in the well- known * Rhomb - porphyry ' of the Christiania district, Norway.
Albite (Na.Al.Si3.O8) is a common felspar in some granites, gneisses, schists, and porphyrites; it is usually rather opaque white, sometimes pearly and opalescent.
Albite very often occurs in parallel intergrowth with orthoclase (Fig. i). This is called the ' perthite structure ' (from Perth, Ontario) ; it may be visible with the naked eye, or only distinguishable with aid of the microscope (micro-perthite); or its presence in an ultra-microscopic condition may be assumed, as in anorthoclase, when it is called ' crypto-perthitic structure.'
Anorthite (CaAl2Si2O8), the lime felspar, is a rare mineral. The plagioclase felspars, intermediate between albite and anorthite, the albite-anorthite mixtures, are important rock-formers. They rarely assume the perfec-
22 GEOLOGY OF BUILDING STONES
tion of form seen in albite, anorthite, or the orthoclastic felspars, but occur rather in irregular grains, stout prisms with ill-defined outlines, or as elongated, lath - shaped bodies. They cannot be distinguished in the hand speci- men, but by one or other of their optical characters they may be differentiated without much difficulty under the microscope. Frequently, however, it is more satisfactory to examine crushed fragments of the mineral rather than the thin slice of the rock.
The more basic plagioclases (those with less silica) are naturally found most commonly in the more basic rocks. One of these, labradorite, in the rock found on the coast of Labrador, deserves attention because its beautiful irides- cent colours in shades of blue, green, yellow, and purple, have led to the stone being employed in small slabs for ornamental objects. The iridescence of the rock is due to both of the essential minerals composing it— namely, hypersthene (p. 27) and labradorite felspar. The irides- cence is caused by minute inclusions of haematite, gothite, ilmenite, or diallage, arranged in a lamellar manner within the crystals. This rock is known as * labradorite ' or ' labrador-spar.' The so-called ' labrador ' or ' labrador- granite ' of Scandinavia is not the same kind of stone (p. 84) ; it has acquired the name from its superficial resemblance to the original rock.
In some rocks the place of felspars is taken more or less completely by the felspathoids, Leucite, KAl(SiO3)2, and Nepheline, K2Na6Al8Si9O34. The former mineral is pseudo-cubic, and the latter is hexagonal ; the one com- monly forms icositetrahedra, the other hexagonal, prisms. Nepheline in the older rocks is frequently granular, and is called Elaeolite, from its greasy appearance.
Mica Group. — This group embraces a number of aluminous silicates of variable composition, but with
MINERALS 23
closely related properties. They crystallize in the mono- clinic system, but, except in certain lavas and in pegma- tites, they do not often develop their perfect form, which is that of nearly true hexagonal plates or pyramids. They are best characterized by their extremely perfect basal cleavage and by the elastic nature of the thin cleavage flakes. From the chemical point of view there are two distinct classes of micas :
1. Alkali micas —
Potash mica = Muscovite 1 For convenience
Soda mica =Paragonite often called
Lithia mica =Lepidolite J 'white mica.'
2. Ferro-magnesian micas —
Iron-magnesian mica — Biotite 1 For convenience
With much magnesia =Phlogopite often called
With much ferric iron = Lepidomelane) ( black mica.'
The alkali micas are paler in colour than the ferro- magnesian kinds. The former vary from silvery white to pale yellow, pale green, or pinkish tints; the latter range from pale brown, brownish greens, and reds, to black.
There are many other varieties with more or less well- defined individuality, but for the present purpose it is sufficient to class all the micas, according to their appear- ance, as ' white ' or ' black.'
Most important are the two kinds — muscovite (and its varieties) and biotite (and its varieties).
Muscovite — potash mica — has a composition which may be represented by the formula H2K.Al3(SiO4)3. H. = 2-3. Sp. gr. = 276-3-1.
It is not attacked by HC1, and, as regards weathering and other forms of alteration, it is one of the most stable minerals. It is found in many granites and gneisses and mica schists; in quartz porphyries, but not in recent erup- tive rocks. In sandstones, grits, and shales, it often plays
24 GEOLOGY OF BUILDING STONES
a prominent role, and, by the tendency of the flakes to lie with their flat faces in one plane, it has a marked effect in emphasizing the fissibility of the stone along bedding planes. Besides occurring as an original mineral in granites, it is frequently found as one of the alteration products of felspar and other aluminous minerals. The varieties known as sericite and Liebenerite often occur in this relationship.
Biotite. — The composition of biotite may be represented approximately by K2HMg6Al3(SiO4),3 ; but iron and other substances are present. H. = 2*5-3. Sp. gr. = 2'8-3*2.
This mineral is a common constituent of many granites, gneisses, syenites, diorites, andesites, and trachytes, also of some basalts and many metamorphic schists.
In igneous rocks and metamorphic rocks whose flow structure or gneissose structure is strongly developed, the presence of abundant mica flakes may tend to produce weakness in a stone in a direction parallel to that of the flow or schistosity. Mica has little influence on the strength of unfoliated rocks or those without marked orientation of the grains. It is, however, a source of trouble if it occurs very abundantly or in large flakes in a stone that has to be polished, for the flakes themselves do not take the polish, but tend to drag out in the process and leave hollows.
Dark micas, especially those rich in iron and magnesia, are much less stable than the light kinds. They pass readily through a series of bodies of indefinite composition, ' vermiculites/ into chlorites. For example :
Biotite. Vermiculite. Chlorite.
Al2Mg2KH.(Si04)3. Al2Mg2H2(Si04)33H20. Al2(MgOH)4. H2(SiO4)3.
The chlorites resemble the micas in appearance. They form similar ragged scales or hexagonal plates; their
MINERALS
colour is usually dark green, and their lustre is inclined to be pearly. They crystallize like mica in the monoclinic system. Unlike mica, the flakes are flexible but not elastic. The composition is very variable ; it resembles that of the micas, but is more strongly hydrated and more basic, and alkalies are often completely absent. They may arise in many kinds of rocks, from secondary changes wrought upon various aluminous minerals besides dark mica — e.g., amphiboles, augite, epidote, garnet, tourmaline.
The green colour of some granites is due to this mineral, and it is the cause of the same tint in many other rocks ; it is especially abundant in certain schists.
Very important rock-forming minerals are the Am- phiboles and Pyroxenes, two groups of silicates which are intimately related both by their chemical and physical attributes. The more prominent members are included in the following list :
AMPHIBOLES.
Anthophyllite :
(MgFe)Si03 Tremolite :
CaMg3(Si03)4 Actinolite :
Ca(Mg,Fe)3(Si03)4 Hornblende :
Ca(Mg,Fe)3(Si03)4
CaMg2Al2(Si(V3 Glaucophane :
NaAl(SiO3)2.MgSiO3 Riebeckite :
NaFe(SiO3)2.FeSiO3
Enstatite :
(MgFe)Si03 Hypersthene :
(MgFe)SiO3 Diopside :
MgCa(SiO3)
CaMg(Si03)2 MgAl2Si06
PYROXENES.
1
Orthorhombic
Acmite or
NaFe(SiO3) Rhodonite :
MnSi03
Monoclinic
Anorthic
Most of the amphiboles are monoclinic, and are char- acterized by a well-marked cleavage parallel to the prism faces; hence in the sections normal to these faces the
26
GEOLOGY OF BUILDING STONES
cleavage cracks appear crossing at angles of 56° and 124° (Fig. 2). The coloured varieties are strongly pleochroic. Hornblende (H. = 5-6. Sp. gr. = 2-9-3), the most common form, is an important constituent in many syenites and granites (monzonites), in which it may be brown or green ; in gabbros, diorites, phonolites, and trachytes, it is usually
FIG. 2.
a, Cleavage in Hornblende ^ at right-angles and parallel
b, Cleavage in Augite / to the prism faces.
c, Cleavage in Mica : a crystal and an irregular flake.
d, Vacuoles in Quartz : highly magnified (left) ; in streaks less
highly magnified (right).
green ; in andesites it is brown. It likewise appears in many gneisses and schists.
Actinolite forms green needles and bladelike crystals in certain schists.
Tremolite is green to white, and occurs in elongated narrow crystals. The tendency to elongation is very marked. The true asbestos is a variety of this mineral in which the fibres (crystals) may attain the length of a yard ;
MINERALS 27
they are silky and flexible. Jade or nephrite is a compact form of the same mineral, or of actinolite.
Riebeckite is a blue mineral ; it is found in some granites, micro-granites, and trachytes.
Important rock-forming minerals occur in the Pyroxene group in both the monoclinic and rhombic systems. The coloured varieties may or may not be strongly pleochroic, and the cleavage parallel to the prism face is not so well marked as in the amphiboles ; the traces of the cleavage plane form angles of 87^° in cross-section, but there is in addition a fairly perfect pinakoidal cleavage (Fig. 2). In the well-formed crystals the prism faces are less well developed than the pinakoids.
Among the rhombic pyroxenes, the minerals Enstatite, Bronzite, and Hypersthene, may be mentioned. They are similar in composition, but differ in the amount of iron they contain.
Enstatite contains less than 5 per cent, of FeO. Bronzite contains 5 to 14 per cent, of FeO. Hypersthene contains about 14 per cent, of FeO.
Bronzite and hypersthene are very prone to a type of alteration which produces the appearance known as ' schiller ' (schillerization). Its effect is mainly due to interference of the light caused by minute layers of hydroxides or oxides of iron deposited in closely adjacent cleavage planes or in glide planes ; or to cavities in the same positions.
These minerals are found forming short prisms or large irregular plates in diorites, some granites (monzonites), dolerites, porphyrites, andesites, and in gabbros and peridotites.
Augite (H. = 5-6. Sp. gr. = 3*2-3-6), the most impor- tant monoclinic pyroxene, often forms well-shaped crystals
28 GEOLOGY OF BUILDING STONES
in volcanic rocks (andesites), or large irregular plates in more crystalline rocks and some basaltic rocks (gabbros, dolerites). It is often difficult to distinguish in the fractured rock from dark hornblende ; its colour viewed in this way is black, but in the thin section it is readily recognized by its optical characters and cleavage. In thin sections it is pale green to pale brown or violet- brown ; pleochroism is feeble or absent.
or Acmite is green in colour ; it occurs in some syenites, nepheline syenites, and phonolites.
In many gabbros augite occurs in an altered condition, resembling that of bronzite and hypersthene ; it is then called Diallage,|and is often laminated (parallel to the ortho- pinakoid), due partly to interlamination with bronzite.
An interesting type of alteration, to which the name Uralitization is given, is the transformation of an augite crystal more or less completely to hornblende- The form of the augite is retained, but the cleavage and composition are those of hornblende. This change, known as ' uralitization,' is usually accompanied by the formation of calcite and epidote in the immediate neighbourhood (see Table III.).
It is most essential to observe that these analyses do not stand for anything more than examples; they are neither average results nor extremes, and in some of the minerals — the dark micas, for instance — very wide divergencies from the above composition are to be found.
Olivine is a silicate of magnesia and iron, (MgFe)2SiO4, in which the magnesia is liable to considerable variation. H. = 7. Sp.gr. = 3'3-.j. It has imperfect cleavage, conchoidal fracture, and vitreous lustre. Usually it appears as greenish or reddish glassy grains in basic rocks (gabbros, peridotites, dolerites) ; in some basalts it forms short prisms, which appear hexagonal in section. This mineral
MINERALS
29
alters very readily along internal cracks and round its margins into serpentine, often accompanied by the liberation of iron, which gives it a red, brown, or black stain.
Serpentine (H4Mg3Si.2O9. H.-3. Sp. gr. = 2'6) is a massive or fibrous mineral with greasy lustre ; green, red, or yellow, in colour. It is formed by the decomposition of
TABLE III.
|
A. |
B. |
c. |
D. |
E. |
F. |
|
|
Silica— SiO2 |
45^3 |
47-06 |
39-48 |
4376 |
43'42 |
47'95 |
|
Titanium oxide — TiO2 ... |
4-28 |
1-82 |
0-30 |
0-78 |
||
|
Alumina — A12O3 ... |
773 |
777 |
12-99 |
11-62 |
19-00 |
30-26 |
|
Chromium oxide — CrO3... |
trace |
— |
— |
— |
||
|
Ferric oxide — Fe2O3 Ferrous oxide — FeO |
2'95 4-07 |
1-30 »-i5 |
7'25 1073 |
6-90 10-47 |
17-64 |
2'43 3-10 |
|
Manganese oxide — MnO |
0*07 |
O'2O |
I -00 |
0*50 |
— |
|
|
Magnesia — MgO |
12-25 |
I3-52 |
11-47 |
12*63 |
°'54 |
0-94 |
|
Lime— CaO ' |
23-37 |
!9'33 |
I2'OI |
9-84 |
1-81 |
0-98 |
|
Sodium oxide — Na2O |
0-47 |
o*33 |
170 |
3'43 |
3-66 |
2'00 |
|
Potassium oxide — K2O ... |
0'12 |
O'll |
2-39 |
1-28 |
877 |
IO'I9 |
|
Nickel oxide— NiO |
005 |
trace |
— |
— |
||
|
Phosphorus pentoxide — |
||||||
|
PA |
— |
0*06 |
— |
— |
— |
— |
|
Water— H2O Fluorine — F |
0'37 |
0'20 |
0-05 |
1-82 |
4-30 |
I-I3 |
|
100-96 |
99-85 |
IOO'25 |
100*49 |
99-14 |
IOO-25 |
A and B = Augite ; C and D = Hornblende ; E = Dark Mica (Lepidomelane) ; F- Light Mica (Muscovite).
several kinds of ferro - magnesian minerals. It is an important feature in many greenish marbles (serpen- tinous marbles), often with associated talc and magnesite. A very markedly fibrous variety is known as Chrysolite, a mineral which is largely exploited as ' asbestos ' (Canadian asbestos).
30 GEOLOGY OF BUILDING STONES
Another basic magnesium silicate is Talc— H2Mg3(SiO3)4. H. = T. Sp. gr. = 27. Like serpentine, it is one of the final decomposition products of ferro-magnesian silicates. When pure it has a silvery-white appearance, but it is very often stained by the presence of from i to 4 per cent, of FeO, and thus becomes green, yellow, or red. It occurs in rocks as small flakes resembling mica, but pliable and non- elastic; also in irregular patches and small veins. It is best known in its massive form Steatite, or Soapstone. Its resistance to fusion and to the attack of acids are noteworthy characters.
A few other silicate minerals may be briefly mentioned here, as they will be referred to later ; they are not, how- ever, of such importance as the foregoing from the point of view of building material formation.
Epidote (HCa2Al3Si3O13. H. - 6. Sp. gr. = 3-4), mono- clinic, is a dark green mineral which owes its colour to a variable amount of iron. It has a vitreous lustre, and in thin sections is strongly pleochroic. It has a good cleavage. It usually occurs in grains or short stout prisms.
Zoisite is a mineral of very similar composition, but orthorhombic in its crystallization. A massive red or pink variety called Thulite, from Telemark in Norway, is used as an ornamental stone.
Both these minerals may occur as alteration products of felspars, and the green colour of some granitic rocks is due to the minute grains of epidote that have arisen in this manner.
Garnets are minerals of the general composition R"R'"(SiO4)3, where R" = Ca,Mg,Fe,Mn, and R'" = Al,Fe,Cr. They crystallize in the cubic system, the common form being the dodecahedrom. Their lustre is vitreous and the fracture uneven; they are readily fusible and brittle.
MINERALS 31
There are several varieties, but it will be sufficient to mention three of the more common ones — namely :
Pyrope— Mg3Al2(SiO4)3 Magnesia garnet.
Almandine — Fe3Al2(SiO4)3 Iron garnet.
Grossular — Ca3A.l2(SiO4)3 Lime garnet.
Garnets are common in some crystalline schists, gneisses, hornfelses, and other metamorphic rocks, and in some basic rocks. In certain limestones they appear as altera- tion products.
Zircon (ZrSiO4) is not uncommon in many coarsely crystalline rocks and their derivatives, sandstones, etc., as a subordinate non-essential mineral, occurring in short prisms and grains.
Tourmaline, a mineral crystallizing in the hexagonal system, is a borosilicate of alumina and alkalies, with more or less magnesia and iron. H. = 7. Sp. gr. = 3*1. It has a subconchonoidal fracture and only imperfect cleavage. Commonly, as seen in rocks, it occurs as black needle-like crystals, short prisms, or irregular grains. In thin slices it is brown or blue, and often strongly pleochroic. Usually it is a subordinate mineral in granitic rocks, and some sandstones, etc. ; rarely, as in the case of luxullianite, it becomes an important part of the rock.
Andalusite (Al2SiO6) occurs in grains or elongated prisms in argillaceous schists and some slaty rocks. A variety with dark enclosures, which give a crosslike pattern in the transverse section, is called ' chiastolite.'
Kaolinite (H4Al2Si2O9), possibly with a variety of minerals of similar composition, forms a considerable part of most clays, and is one of the most common results of decomposition in felspars. The cloudy opacity of the felspars in many rocks is usually put down to this
32 GEOLOGY OF BUILDING STONES
mineral, although it is rarely possible to actually determine it by its physical properties.
Several important rock-forming carbonates may now be considered. The first ofthese is Calcite (CaCO3) , carbonate of lime. This mineral crystallizes in the hexagonal system, and it assumes a great variety of forms; but as a rock- builder it nearly always exists as irregular grains. It possesses a very perfect cleavage in three directions, parallel to the faces of a rhombohedron, and lamellar twinning is prevalent. H. = 3. Sp. gr. = 272. The cleavage planes, along which it nearly always splits when broken, have a vitreous lustre. The colour is white unless the mineral has inclusions, and often it is colour- less and transparent. Calcite builds up the bulk of all limestones and marbles, even in many of those which appear to consist of amorphous carbonate of lime. It appears in granular patches in many igneous rocks, as the result of the breaking down of lime-bearing minerals. In some sandstones it may act as a cement.
Dolomite is a double salt with a composition repre- sented by CaMg(CO3)2. H. = 3'5. Sp. gr. = 2'8s. Like calcite, it has a perfect rhombohedral cleavage, and it crystallizes in the same system. Its lustre is pearly, and it is usually rather yellower than calcite, although some dolomite marbles are beautifully white. It occurs in many limestones as isolated rhombohedra and irregular grains ; also forming massive bedded rocks.
Chalybite (FeCO3. H. = 3'5. Sp. gr. = 3*8) forms crystals much resembling dolomite, but they are usually darker. It occurs as scattered crystals or in groups in many clays and some limestones. Sometimes it is found in the form of small spheroidal nodules in clays (sphcero-siderite). It also occurs in veins and in some sandstones as a cement- ing material.
MINERALS 33
Magnesite (MgCO3) is not commonly a rock-forming mineral ; it occurs in veins in serpentine and other decomposed magnesia-bearing rocks and in massive beds. It is a white opaque mineral with conchoidal fracture. It is employed in the manufacture of refractory bricks.
Iron-bearing minerals are not present in bulk in build- ing materials, but they are so widely spread throughout all kinds of rocks, and in their oxidized and hydrated state they act so powerfully as colouring agents, that several of them must be noticed here.
Magnitite (Fe Fe2O4) is a black opaque mineral with a metallic lustre; it crystallizes in the cubic system, usually forming octahedra or dodecahedra.
Haematite (Fe2O3) is black, iron-coloured with brilliant metallic lustre, or red, brownish - red, and earthy. It crystallizes in the hexagonal system. H. = 6. Sp.gr. = 5*2.
Limonite (2Fe2O3.3H2O) is an amorphous brown or black mineral. H. = 5'5. Sp.gr. = 3*8.
Gb'thite (Fe2O3.H2O) is blackish-brown, and crystallizes in the orthorhombic system. H2. = 5. Sp. gr. = 3'8-4'4.
Pyrites— Iron Pyrites (FeS2. H. = 6. Sp. gr. = 5-1) occurs as cubes, octahedra, or pyritohedra, and as irregular grains and masses of a brass-yellow colour and bright metallic lustre.
Marcasite (FeS2. H. = 6. Sp. gr. = 4'8) has the same composition as pyrites, but it crystallizes in the ortho- rhombic system, and its prevalent habit is that of radiating fibrous nodules. When fresh its colour is pale yellow, but it decomposes with great readiness, giving rise to a white efflorescence (ferrous sulphate). This tendency to rapid decay causes its presence in building material to be very harmful.
3
34 GEOLOGY OF BUILDING STONES
Well-crystallized pyrites withstands weather very well, and is much less harmful ; but when it does decompose, it forms the injurious sulphuric acid and the yellow or brown stain of the oxides and hydrates mentioned above.
Pyrites is found in many of the coarsely crystalline igneous rocks, in limestones, particularly earthy varieties, and in clays.
Three sulphates are of sufficient interest to be men- tioned here :
Gypsum — Sulphate of Lime (CaSO4.2H2O). Monoclinic. H. = 2. Sp. gr. = 2*3 — is a colourless transparent mineral when in the crystalline form, called Selenite. These crystals are of common occurrence in many clays, where they are found as isolated individuals or in aggregate groups ; to labourers they are often known as ' congealed water.' A brick clay, otherwise quite good, may be rendered unfit for use by their presence. In its massive crystalline form gypsum is called Alabaster ; it may also be stalagmitic or fibrous (satin spar) in veins.
Plaster of Paris derives its name from the excavations of Montmartre near Paris, where the gypsum beds are worked.
A natural anhydrous sulphate of lime is Anhydrite (CaSO4). Barytes, the sulphate of barium (BaSO4), or ' heavy spar,' is mentioned here because it occasionally forms the cementing material in sandstones.
Ilmenite (FeTiO3) and Titanite (CaSiTiO5) occur as subordinate minerals in many rocks.
Glauconite is a green silicate of iron, with magnesia, potash, and other substances ; of variable composition. In the form of grains and casts of fossils it constitutes an important ingredient in certain rocks — e.g., the Green- sands, Kentish rag, etc.
CHAPTER III IGNEOUS ROCKS
ROCKS may be classified broadly into three groups : (i) Sedimentary, rocks formed by deposition at the surface of the earth of material lying in beds or layers. (2) Igneous, rocks formed by the fusion of rock matter deep within the earth's crust ; often found penetrating the sediments. (3) Metamorphic, rocks of sedimentary or igneous origin that have suffered certain changes in structure and composition, since their first formation.
IGNEOUS ROCKS.
It is not to be supposed that all igneous rocks have penetrated the others in which they lie, by injection from below, for some, like the lavas of existing active volcanoes, may be seen actually hardening at the surface and forming regular beds there. We do not depend upon this criterion to distinguish igneous from sedimentary rocks, for it is possible to recognize the igneous origin of rocks even in hand specimens and microscopic fragments from their peculiarities of internal structure.
An igneous rock is one that has solidified from a molten state, and the variations in the conditions of solidification lead to variations in character which permit a classifica- tion of these rocks into three tolerably well-defined groups :
I. Those that have solidified deep within the earth's crust under what are called Plutonic or Abyssal con-
35
36 GEOLOGY OF BUILDING STONES
ditions — that is, under great pressure and at a slow rate of cooling.
2. Those that have solidified in conditions of more rapid cooling — Hypabyssal conditions.
3. Those that have solidified at or quite near the surface under conditions of comparatively rapid cool- ing, and subject to pressure little above that of the atmosphere.
From this point of view, therefore, we may divide igneous rocks into three broad groups corresponding to the circumstances of their origin :
1. Plutonic or Abyssal rocks — Intrusive (Granite).
2. Hypabyssal rocks — Intrusive (Porphyry).
3. Volcanic rocks — Extrusive (Rhyolite).
Each of these three types of rock has been derived from a molten rock magma, which, as we shall see later, may present many variations in composition from place to place ; indeed, variations may frequently have arisen from time to time within the same magma mass during the process of solidification.
We may now briefly notice some of the modes of occur- rence and more obvious peculiarities of these three types.
The Plutonic rocks, of which granite may be taken as the type, appear at the surface of the earth only as the result of the removal by ordinary agencies of weather- ing of vast masses of superincumbent strata. Thus, they appear in the cores of mountain ranges or in great masses which have been injected under and into overlying beds, sometimes lifting the latter up, as in the injected masses with the form known as ' laccoliths ' ; sometimes being squeezed along planes of weakness in horizontal, vertical, or intermediate directions ; occasionally absorbing some of the surrounding rock, and almost invariably causing
IGNEOUS ROCKS 37
modification (metamorphism) by virtue of their high tem- perature.
Most of the material so injected into the hardened portion of the crust was probably at one stage of its existence in the condition of a structureless fluid, con- taining all the elements of granite, for example, in a state of mutual solution.
As cooling set in, some of these substances crystallized out of solution in the form of definite minerals ; others followed, according to the regular laws of solutions, until the whole mass had crystallized into a solid rock ; the later minerals fitting in between and moulding themselves round those formed at an earlier stage.
In the deeply-seated plutonic masses the cooling must of necessity have been extremely slow, with the result that the whole rock became completely crystalline (holo- crystalline, Plate II., 6), and no non-crystalline, undifferen- tiated material remained. Moreover, the crystalline struc- ture of such rocks is usually coarse, and each of the principal component minerals is well developed.
The Hypabyssal rocks, like those of the plutonic group, are all intrusive in others that have consolidated earlier ; but the bodies of molten material from which rocks of this type have consolidated were smaller than those giving rise to plutonic rocks ; their rate of cooling was conse- quently more rapid.
Individual crystals of earlier-formed minerals may be of comparatively large size, but there is usually a distinct ground mass of much finer grain than is associated with the plutonic type (Plate II., d). This ground mass may be holocrystalline and granular, like that of granite on a small scale, or it may be formed of a microscopic intergrowth of the essential minerals (granophyric structure) ; it may be a densely felted mass of crystalline material in which
3$ GEOLOGY OF BUILDING
he individual crystals are often too small to be dis- inguished (felsitic structure), or it may be glassy.
Two or more of these varieties of ground mass may be associated within the same rock.
Rocks of this type occur as offshoots from the great plutonic masses; they may even appear on the outer, rapidly chilled surfaces of such masses. They occur also in the forms of veins or dykes, forced across the bedding of stratified rocks or through joints in earlier - cooled igneous rocks ; and they appear as bedlike sills injected between the beds of stratified rocks ; or they may form small bosses and laccoliths.
The Volcanic rocks differ from those in the other two groups in that they are always extrusive in character ; they have been poured out or blown out at the surface of the earth or on the bottom of the sea. They are repre- sented by modern lava flows and the outpourings of so- called volcanic ' ash,' pumice, and similar fragmentary material of various sizes, and by their ancient equivalents formed by the volcanic activity of previous geological periods.
Such rocks usually occur stratified as bed upon bed of lava or of volcanic ash (tuff). Beds of lava may be inter- stratified with those of tuff, and either form of volcanic deposit may lie between strata of the ordinary sedimentary kind.
Naturally, the rate of cooling in these circumstances has been more rapid and irregular than in the case of the two other groups. A large proportion — sometimes the whole of the rock mass — is found in a glassy or only feebly devitrified condition (Fig. 3, V).
The free movement of the rapidly cooling rock has frequently left its mark in fluidal or flow structures, and the relief from pressure on extrusion has often given
VI
FIG. 3.— SOME STRUCTURES IN IGNEOUS ROCKS.
I. Granite: quartz, clear; felspar, clouded; mica, dark. II. Granite porphyry : a ground mass of the same minerals as in (I.), with similar structure, but finer grained ; porphyritic quartz, plagioclase, ortho- clase, and mica (polarized light). III. Diabase or dolerite : typical ophitic structure ; large crystals of augite enclosing crystals of plagio- clase. IV. Basalt : granular crystals of augite with lath-shaped felspars ; porphyritic olivine, cracked and stained ; some residual glass and small grains of magnetite. V. Obsidian : a glassy volcanic rock ; streams of microlites (undeveloped crystals) showing the flow struc- ture ; curved ' perlitic ' cooling cracks. VI. Trachyte: showing flow structure ; large crystals of sanidine felspar in a ground mass of smaller crystals of the same.
40 GEOLOGY OF BUILDING STONES
rise to vesicular structures, so well exemplified in the frothy nature of pumice.
There is a type of texture or internal structure common to the three rock groups, which is of considerable impor- tance, since it materially modifies the appearance of the stone. It is due to the crystallization of one or more of the characteristic minerals of the rock at an early stage, so that it had freedom to assume its proper form with well-developed crystal faces. These well-formed (idio- morphic) crystals or phenocrysts are said to be porphy- ritic ; the rock, too, which contains such crystals is called a ' porphyritic ' rock. Several of the minerals in a single rock may be porphyritic, and occasionally a single mineral may appear in two distinct crops of phenocrysts. Porphy- ritic structure is less prevalent in plutonic rocks than in the two other types.
The classification of igneous rocks may be undertaken from an entirely different standpoint — namely, that of their chemical composition. They all contain silica, alumina, and varying amounts of potash, soda, lime, magnesia, and oxides of iron. The silica exists in the free state as the mineral quartz and in a variety of com- binations with the other constituents, and it has been found convenient in practice to recognize a grouping based upon the total silica contents of the rocks. Thus we have an Acid group, with 66 per cent, or more of silica; an Intermediate group, with silica varying from 52 to 66 per cent. ; and a Basic group, with the silica con- tent below 52 per cent. This simple relationship is compli- cated, however, by the fact that in a given magma earlier- solidified portions are frequently more basic than what is left over to solidify later ; therefore apophyses formed at this period may be basic, while those found later may be acid.
IGNEOUS ROCKS 41
The two systems of grouping, one founded on the mode of origin and the other on the chemical composition of the rocks, are quite independent. Examination of any area where igneous rocks of diverse origin are exposed shows that the great masses of slowly cooled plutonic rock have sent out prolongations or apophyses into the surround- ing strata, which have cooled more rapidly than the parent mass, and therefore have the structures peculiar to the hypabyssal type ; and, further, when these tongues have reached the surface and outpoured there, the still more rapidly cooled material has all the characters of the volcanic type.
Here, then, if the parent magma were of the acid variety, as indicated by its chemical composition, we should find plutonic, hypabyssal, and volcanic rocks all of the same chemical type ; conversely, if the parent magma were basic, the associated apophyses and erupted volcanic rocks would likewise partake of the same nature.
Since the specific characters of an igneous rock — those characters by which it is distinguished from other igneous rocks — are determined by (i) its chemical composition and (2) its internal structure or fabric, both of these factors have to be taken into consideration simultaneously in any scheme of classification for practical purposes.
An igneous rock is an aggregate of discrete minerals together with more or less undifferentiated matter. The kind of rock is recognized by the nature of its component minerals, by their mutual relations with one another in the aggregate, and by their relative proportions in the rock.
The nature of the minerals — their specific character and peculiarities of habit — is not fixed solely by the chemical composition of the original liquid magma, but by
42 GEOLOGY OF BUILDING STONES
a very complex and variable set of conditions, which include changes in pressure, in temperature, and in the relative masses of different compounds in solution at any given moment during the cooling process, to which must be added the introduction of fresh magma streams, of the same or slightly different composition, into that already partially crystallized.
The chemical analysis of an igneous rock may tell us whether it should be classed as acid, intermediate, or basic; or in one of the innumerable gradational stages between one or other of these divisions; it will tell us whether the rock has a high or low percentage of potash, soda, magnesia, iron, and so on, but with all this we shall know nothing of the rock as a rock.
It is not sufficient to know the chemical composition ; WTC must know also the minerals of which the rock is composed, and the manner in which they are mutually associated.
Since the composition of the rock-forming minerals is well known, it is often enough for us to recognize in the hand sample, or in thin sections under the microscope, their kinds and relative proportions, in order to be able to form a very fair idea of the composition of the rock. Chemical analysis is necessary to confirm and give exacti- tude to such observations, and it is, of course, essential for the determination of the nature of the undifferentiated, crypto-crystalline, or glassy matter which cannot be resolved by the microscope.
In Table IV. a small number of the better-known rock types are arranged in such a way as to show roughly their relationship, both genetically and chemically. Petrologists have recognized and described very many other types to which distinctive names have been given ; but the student should clearly understand that all these divisions are artificial, and though some names are necessary for con-
IGNEOUS ROCKS
43
venience, many of the names in the literature are ill-chosen and of trivial signification. The rocks themselves show no such marked individuality of type except locally, for even the most characteristic plutonic rock is found in Nature to pass by insensible gradations into those that would be recognized as hypabyssal or volcanic ; and chemically there is no sharp line to be drawn anywhere between the most acid and the ultrabasic igneous rock.
TABLE IV.
Acid. Intermediate.
Baste.
|
Orthoclase Felspar. |
Plagioclase Felspar. |
|||||
|
With |
Without |
Hornblende |
Augite and |
|||
|
Quartz. |
Quartz. |
or Angit<-. |
Olivine. |
|||
|
Completely crystal- line; coarsely grained, sometimes |
utonic |
Granite |
Syenite |
Diorite |
Dolerite Gabbro |
|
|
porphyritic |
J* |
|||||
|
Completely crystal- |
I |
|||||
|
line, or with small |
'rt |
|||||
|
proportion of un- |
O >^ |
|||||
|
crystallized glassy material; fine- |
v o |
H -t |
Rhyolite, Quartz- |
Syenite- porphyry |
Andesite |
Basalt |
|
grained ground |
0 |
porphyry |
||||
|
mass, sometimes |
^ |
|||||
|
with porphyritic |
||||||
|
crystals |
' |
|||||
|
Glassy, non-crystal- line |
111 f $ 1 |
Obsidian Pitch- stone |
Trachyte- glass Pitch - |
Andesite- glass |
Tachv- lyte |
|
|
) > I |
stone |
44 GEOLOGY OF BUILDING STONES
GRANITE.
Granite, as its name implies, is a rock made up of granular particles. These grains are crystalline individuals of three or more kinds of minerals ; they are united firmly together by their intergrowth to form a rigid mass.
Granites exhibit many variations of colour, texture, and strength, but all true granites are built up of the same limited number of mineral species. The essential minerals forming granite are felspar, quartz, and mica; frequently with hornblende, and sometimes with augite in place of, or in addition to, the mica.
Many totally different kinds of rock have been called ' granites ' — sometimes from ignorance of their true nature, sometimes for trade purposes ; thus, one hears of ' Mendip Granite,' which is in reality a limestone; ' Petit Granite,' a dark Belgian limestone ; * Ingleton Granite,' a con- glomerate ; ' Black Granite,' a name applied to many varieties of basalt, diabase, and other basic igneous rocks, and so on. But in the interests of everyone concerned, it is desirable to employ the name only with its proper significance.
The Minerals forming Granite.
Felspar. — This is the most obvious of the granite minerals, and in the majority of granites is the most abun- dant. Its colour — white, grey, yellow, pink, or red — together with its opacity, renders it easily distinguishable in the freshly fractured or polished surface of any granite. The felspar of granites is very rarely transparent, as it is occasionally in a few varieties of volcanic rock, though it may be translucent.
The felspar of granite is not all of one kind ; while the most prevalent form is orthoclase, it is very frequently associated with a plagioclastic variety. In typical granites
IGNEOUS ROCKS 45
the potash felspars predominate, in the form of orthoclase, microcline, or perthite ; more rarely a soda-felspar, soda- orthoclase, albite, or anorthoclase, may assume pre- dominance. Rocks of the latter character have been called ' alkali-granites.' Again, in some granites the soda- lime felspars are associated with alkali-felspars in about equal proportions (Adamellites). Finally, there are granites in which the soda-lime felspars exceed the alkali- felspars in abundance (Grano-diorites), It is important to remember that these distinctions cannot be recognized by the unaided eye.
The form taken by felspar in the majority of granites is that of irregular grains, either without any indication of crystalline form or with its faces only partially and imperfectly developed. In many granites the felspar grains are grouped in clots or aggregates. It sometimes happens that some of the felspar occurs in the form of tabular crystals : small, % inch to f inch, as in Penryn ; or large, I inch in Shap granite, I to 5 inches in Lamorna or Colcerrow granite. These well-formed crystals constitute an earlier crop of felspars, which has usually been suc- ceeded by a later crop of ill-developed grains. Quite frequently the earlier porphyritic crystals are orthoclase or perthite, while the later-formed grains are albite or oligo- clase or orthoclase. Occasionally there may be two crops of porphyritic crystals, larger and smaller.
The colour of granitic felspar has a fairly wide range of variation, as indicated above. As a rule the orthoclase crystals are more prone to coloration than the plagio- clases, the latter being very commonly dead white, pearly white, or light grey. The pink, yellow, buff, and red tints in orthoclase are all due to the inclusion of very minute granules of iron oxides. This coloration may be an original character of the felspar, or it may be, in the case of much-weathered granites, a rusty stain introduced from
46 GEOLOGY OF BUILDING STONES
without. Green tints in felspar, exemplified in the granites of Lamorna and other Cornish examples, is usually due to the presence of minute scales of chlorite ; in other cases it is caused by granules of epidote produced during the partial decomposition of the felspar.
The lack of transparency which characterizes the felspar of nearly all granites is due in part to the frequency of cleavage planes, and in part to the presence within the crystals of minute granular decomposition or alteration products ; in either case the effect is the same — namely, the repeated reflection of the incident light from the numerous surfaces so formed.
Quite apart from the processes of ordinary weathering, the felspars of granites have usually undergone some form of alteration whereby more or less of the original felspar crystal has been transformed to an aggregate of other minerals.
Quartz in granites almost always takes the form of irregular grains, mostly commonly moulded upon the other minerals ; sometimes it occurs in rounded blebs, and more rarely some of the crystals are found with an approach to the perfected crystal outline.
It sometimes happens that the quartz and felspar in portions of a granite mass, more frequently in granite veins, have crystallized simultaneously, either mutually interfering with one another or forming parallel growths, as in ' graphic granite '; this may take place either on a large scale, visible to the naked eye, or on a microscopic scale, when it is known as ' granophyric ' structure (p. 51). Most of the quartz of granite is clear and glasslike, and for this reason it usually appears dark grey in the fractured or polished surface, since the light which enters it is absorbed by the surrounding minerals, and little is reflected. Some granitic quartz is tinted dark grey, ' smoky quartz,' or it may be faintly yellow, pink, violet, or bluish.
IGNEOUS ROCKS 47
There is no distinct cleavage in granitic quartz, but its place is taken to some extent by the multitude of extremely small cavities, which tend to lie along certain planes within the quartz grains. Frequently it will be observed that within a block of granite there is a uniformity of direction among the planes containing cavities in the majority of the quartz grains ; thus for example, we may find one series of approximately parallel planes with a fairly con- stant north and south orientation, while these are inter- sected by another series running east and west, or horizontally. The possible significance of these cavities in the process of quarrying will be noted farther on. The cavities have been briefly described on p. 17.
Mica. — The mineral which has taken the third place in order of importance in granites is mica. It will be suffi- ciently accurate for our present purpose to recognize two kinds of mica, dark and light. The former is present in the great majority of granites ; the latter is comparatively rarely found alone, but is frequently associated with the dark kind ; sometimes the two are united in parallel growth. The dark mica is dark brown in colour; it is commonly called Biotite, a term which here embraces more than one variety of the mineral (haughtonite, con- taining much ferrous oxide ; lepidomelane, with much ferric oxide). The light mica is often called Muscovite. Mica is usually present in the form of small ragged crystalline plates, readily distinguished in the fractured or polished rock by its colour, irregular outline and softness.
Rarely, granitic rocks are found quite free from mica, consisting only of felspar and quartz.
The minerals which sometimes more or less completely take the place of mica in granite are hornblende, augite, enstatite, and tourmaline. They are all dark in the hand specimen, and not easily distinguishable without the
48 GEOLOGY OF BUILDING STONES
aid of a lens. Granitic hornblende in thin sections is brownish - green or green, showing the characteristic cleavages ; granitic augite is usually pale - coloured in microscopic sections ; and enstatite is also pale when examined under the same conditions, and is rarely very abundant. The tourmaline in thin sections is blue or brown ; some of the Cornish granites (luxullianite) contain large quantities of the mineral.
The essential mineral constituents of granite are those already mentioned — namely, felspar, quartz, and mica or its representatives. In addition to these there are many others — accessory minerals — quite subordinate in bulk, though of great interest to the petrologist ; their influence upon the behaviour of the stone as applied to structural purposes is insignificant, and the mention of the names of a few of them will suffice. The accessory minerals include magnetite, pyrite, haematite, apatite, sphene, andalusite, cordierite, and zircon ; tourmaline may be included here.
Certain other minerals are met with in granites that have undergone more or less decomposition ; these will be noticed in the section on weathering.
Texture. — The coarseness or fineness of grain, the disposition of the several minerals forming the rock, and their mutual relationships, are the characteristics embraced in the term * texture ' (' fabric ' or ' structure ').
To obtain a complete knowledge of the texture of a granite, it is essential that its appearance should be examined in large polished slabs, and in thin slices in the microscope ; and these aspects of the stone should be com- pared with the characters exhibited upon a freshly fractured surface. If attention is confined to one only of these methods of examination, a very misleading conception of the texture of the stone is likely to result.
PLATE II
IGNEOUS ROCKS.
a, Gneiss, Scotland ; b, Granite, Cornwall ; c, Granite, fine-grained, with pegmatite vein showing the same minerals on a large scale ; d, Porphyritic structure, Porphyrite, Cumberland; e, Andesite (both black and white specks are mica scales).
IGNEOUS ROCKS 49
Size of Grain. — There is possibility of considerable latitude in the size of grain in granites ; as a general rule there is a fair amount of concordance among the three important minerals in this respect : if the felspars are large, the quartz and mica tend to be large also, although there are exceptions in which quartz, for instance, is pro- portionately larger than usual.
At the outset it is necessary to distinguish two types of texture — the non-porphyritic, or typical granitic texture, and the porphyritic type, in which one or more of the felspars stands out prominently from the rest by reason of its greater size, frequently associated with greater perfec- tion of form. Large porphyritic felspars may be developed in a crystalline ground mass which is either coarse- or fine-grained, but it is obvious that a stone of the latter kind will act under stresses more like a coarse-grained than a fine-grained one.
It is sufficient for our purpose to estimate the size of grain of non-porphyritic granite by reference to the felspars. Dale regards as coarse a granite containing felspars of i centimetre, or -f inch, and over ; in a medium-grained granite they will be between 0*5 centimetre (J inch) and i centimetre (f inch) ; while in a ym^-grained granite they fall below this limit. Many of the granites in constant use are coarser than the limit indicated above, and granites occur in which the grains are very small, 0*175 centimetre (about 0*007 inch), but such fine-grained granites are not much used.
Porphyritic crystals range in size from J inch to 5 inches; examples of this magnitude may be observed occasionally in the Colcerrow stone.
In estimating the size of individual grains, either of quartz or felspar, the fact is very often ignored that the apparently homogeneous grains are themselves formed of an aggregate of distinct crystalline individuals in close
4
50 GEOLOGY OF BUILDING STONES
juxtaposition. This can always be readily verified in the thin slice under the microscope. Bearing this in mind, it is evident that some granites are much finer-grained than they appear.
Mutual Relations of the Minerals. — It has previously been stated that in a granite all the minerals are crystalline ; there is no glassy or non-crystalline material ; moreover, with the exception of the porphyritic felspar and some of the small minor accessory minerals, it is exceptional for the individual mineral grains to have perfected crystal outlines. The crystalline grains have formed in situ — not simultaneously, but in regular sequence, the more basic minerals first, followed by the others in order of decreasing basicity.
Thus, the normal order in most plutonic rocks is ap- proximately as follows : First appear the small accessory minerals, magnetite, apatite, zircon, sphene ; then the ferro - magnesian minerals, augite, hornblende, biotite, muscovite ; these are followed by the felspars, from the more basic lime-felspars through the soda-lime, to the soda and potash varieties ; finally comes quartz and, if it is present, microcline.
This order holds good in a large proportion of granites, but partial reversals of the normal sequence are not un- common ; for example, orthoclase may be found enclosing grains of quartz — hence later — as in Shap granite and some Cornish examples ; muscovite may precede biotite, as in Rubislaw granite ; orthoclase may precede albite, as in some Cornish granites ; and so on.
The texture of granite is frequently influenced by the simultaneous crystallization of pairs of minerals. What appears to the unaided eye as large simple crystals of orthoclase are in reality intergrowths on a microscopic scale of that mineral with one of the more basic felspars. Similarly, biotite and muscovite are occasionally found
IGNEOUS ROCKS 51
intergrown. Again, felspar and quartz may have grown together, either on a coarse scale, as in 'graphic granite* (Plate I.), or on a microscopic scale, as in ' micro-graphic ' (granophyric) or micro-pegmatitic structure. Here it is interesting to note, as throwing light on the intimate bonding of granitic minerals, that what appear as small isolated specks of quartz in the midst of a felspar crystal are in fact often the portions of large individuals seen in section. It is as though a sponge-like framework of felspar had been filled and surrounded by a single quartz individual.
In general, among granites suitable for constructional purposes, the distribution of the minerals is very regular and constant throughout large masses. The grains are commonly roughly equidimensional, and where they have unequal dimensions (as may be in the felspars and mica) they are arranged haphazard without definite orientation. On the other hand, there are few granite masses which do not exhibit a tendency to orientation of the elongated or flat grains in some portion or other of the mass. The micas may tend locally to lie with their flat faces all in one direction, and the same may apply to the tabular felspars. Such orientation is due to flow movements in the partially consolidated mass, and is known as 'flow structure.' This tendency, developed among the pale tabular felspars in some Cornish granites, produces in the quarry faces and in large polished slabs the beautiful effect of eddies and drifting in falling snow. A slight tendency in this direction is observable in the flesh-coloured por- phyritic crystals in large slabs of Shap granite. Flow structure in non-porphyritic granites is best made evident by the mica flakes.
Rarely, granites exhibit an orbicular or spheroidal arrangement of the minerals similar to that described on p. 86 (napoleonite), and occasionally the minerals are
52 GEOLOGY OF BUILDING STONES
found roughly arranged in bands. Irregularities in the normal texture of granites may arise from the following causes :
1. Sudden changes in the coarseness or fineness of grain may appear either in the form of irregular rounded clots in the midst of the uniform granite, or as veins traversing the mass. These clots or veins (pegmatitic veins) may or may not consist of the same minerals as the surrounding mass.
2. Sometimes cavities occur (geodes, druses, vughs), which are lined writh perfectly-formed crystals, quartz, felspar, mica, similar to those in the surrounding rock.
3. Dark inclusions, xenoliths (' heathen ' of quarrymen) are sometimes local aggregates of the darker, more basic minerals ; sometimes they are included portions of the rock into which the granite has been protruded, much altered, of course, by the heat.
Characteristics of Granite in the Mass. — Certain features which are common to most granite masses in all parts of the world may be noticed here ; they are joints, rift and grain, veins and dykes. These structural characters are best observed in quarries, where their presence exerts a considerable influence upon the method of getting the stone.
Joints. — The joints in granites are of two kinds — vertical and horizontal. The former are divisional planes which run through the rock either vertically or inclined at a very high angle to the horizon ; they usually extend with great regularity to great depths. It is no unusual thing to see a single joint-face in a quarry 50 to 100 feet high, and apparently reaching to still greater depths. In any single mass of granite these vertical joints are generally constant in direction, and two sets are commonly found approximately at right angles with one another, with only
IGNEOUS ROCKS 53
subordinate deviations from these two directions. These divisional planes in the best part of a quarry are of no breadth, the two adjoining faces of rock being closely adjacent ; but there is often a thin film of iron stain on the faces, due to the passage of water and air within the cleft. Occasionally it is obvious, from the smoothed and slickensided faces of the joint, that movement has taken place along the plane, the rock on one side having been shifted slightly relatively to that on the other side, either vertically or horizontally.
For the greatest convenience in quarrying, these joints should neither be so near together as to break up the rock into blocks too small for use, nor so far apart as to render the abstraction of large blocks a difficulty. Some- times, in place of a single clearly defined joint, the granite is traversed by a series of closely adjacent parallel joints ; such tracts are a source of inconvenience in quarrying, since they have to be removed as waste or left standing. The stone in the neighbourhood of these joints is often badly stained or weathered. Highly jointed tracts of this kind are called ' end-grain ' or ' grain-end ' in the Cornish district ; l headings ' in some American quarries.
The second kind of joint is horizontal or nearly so ; by these joints the rock is split up into ' sheets ' or ' beds.' While the vertical joints are strictly analogous with the like structures in sedimentary rocks, the horizontal ones are of entirely different character ; they have no relation- ship with the horizontal bedding planes of the sediments or stratified rocks, although it is usual and convenient to speak of the slabs of rock so formed as beds. They are certainly the result of stresses set up within the mass subsequent to its solidification, but what the nature of the causative stress may be is a problem about which there is considerable diversity of opinion ; it is not even clear whether the cause is the same in all cases.
54 GEOLOGY OF BUILDING STONES
To speak of this kind of joint as * horizontal ' is not strictly correct. When they are well developed in the middle of large granite masses, they are often as near horizontal as may be, and form extensive level floors in the quarries hundreds of yards square ; but more fre- quently they depart from the horizontal attitude to a greater or less extent, and tend to follow with rough parallelism the external form of the granite mass. In all cases, whether they are developed on a large or small scale, they tend to break up the granite into lens-shaped masses. As a rule they are more numerous near the exposed surface of the granite than they are lower down in the mass. The tendency to follow the contour of the granite surface is well exhibited in the Cornish quarry districts and on the borders of Dartmoor, as indicated in Figs. 4.6 and 5, but it may be observed wherever the granite has an undulating surface, and near the borders of granite masses.
Except near the weathered surface, the sheets formed by horizontal joints are seldom thin; they range from 6 inches to 30 or 40 feet.
When joints are running ' tight ' or close together, the effect of weathering has been considerable, resulting in a rotten, friable stone. Tracts of this character are sometimes referred to by the quarrymen as ' sand,' ' sand- bars,' or ' bars ' ; ' heads ' in America. The term * sand- streaks ' or ' sand-seams ' is applied to thin micaceous veins in some of the granite of Concord, New Hamp- shire, U.S.A.
Rift and Grain. — Many granites, though not all, show a marked tendency to split more freely in one direction than another. When this tendency is present, it is due to what is called the ' rift ' of the stone. In some cases the cause of the rift is readily ascertainable ; in others it is very obscure. In granites in which a flow struq-
IGNEOUS ROCKS 55
ture is well marked — that is, when there is a definite orientation of some of the minerals — it is clear that the rift is conditioned by this arrangement. Thus, the mica flakes may all lie parallelwith one another, with their broad faces and cleavage planes in the same direction ; or the felspars may have a similar disposition. In either case both the form of the crystal and its internal cleavage will assist the rock to split.
On the other hand, rift is quite well developed in rocks in which no such regular orientation of the minerals is discernible ; it appears then to be due to the effects of stresses, perhaps similar in kind to those which have pro- duced the joints, but differing in degree, and more diffuse in operation.
It has been demonstrated in the case of certain granites that the rift is parallel in direction to sets of minute, rather irregular cracks, which were observed to traverse several crystals, quartz, or felspar, indifferently, without change of direction. Further, in some granites it seems to be associated with the prevalent direction of the sheets of microscopic bubbles and inclusions in the quartz (see p. 17). These sheets of bubbles are very abundant in some granites, and they are often seen to run in two directions, approximately at right angles, both of which are maintained in an irregular manner as the sheets are traced from one particle of quartz to another.
The ' grain ' of granite is the direction in which the stone may be split with a degree of ease second to that of the rift direction. [The term ' grain ' used in this sense has nothing whatever to do with the coarseness or fine- ness of grain (texture).] Grain is due to the same causes which induce the rift, but they have operated in a lesser degree. The direction of the grain is approximately at right angles to that of the rift.
There are many peculiar features in the disposition pf
56 GEOLOGY OF BUILDING STONES
rift and grain within granite masses which have not yet been clearly explained. For example, the rift may be very constant in direction in granite masses extending over a considerable area; on the other hand, it may vary in closely neighbouring quarries, or it may even be different in direction and degree in adjacent sheets within the same quarry. It may run vertically or horizontally, conforming to the direction of one or other of the three main sets of joints ; or it may run diagonally across from joint to joint ; or in one part of an area it may be horizontal or vertical, and gradually pass over into an intermediate or inclined direction towards another part of the area.
To whatever cause the rift may be due, its influence upon quarrying operations is most marked. Where it is well developed the stone may be split with ease by plug and feather, while in its absence blasting would be impera- tive. This means, of course, a saving both in material and labour. Certain granites cannot be economically fashioned into setts or Klein-pflaster, simply because this property of ready splitting is lacking.
A curious feature in the behaviour of some granites is that the rift is more perfect if the stone is split from one direction than from another. This is illustrated in the diagram (Fig. ^A): if the plugs are placed in the face b, the split runs true and square; but if they are placed in the face a the split tends to leave the right plane, and turns off diagonally or irregularly, as shown by the dotted line.
Besides being of so much importance in quarrying the granite, the presence of rift has a marked influence upon the behaviour of the stone under pressure, although I have never seen an example of a granite block that had suffered in a building through being laid with the rift vertical ; it is, indeed, extremely difficult or impossible to recognize even in old blocks when they are set in position.
The three directions in which blocks are cut in the
IGNEOUS ROCKS
57
quarry usually receive distinctive names from the quarry- men ; thus, they may be designated the ' rift,' the ' grain,'
FIG. 4.
A . Rift and grain in granite.
B. Pseudo-bedding in granite.
C. Diagonal or current bedding in sandstone.
D. Horizontal bedding : rocks of different kinds.
E. Folded strata, originally horizontal.
F. Diagram of the relations of dip and strike.
and the ' hard ' or ' hard-way '; or the ' cleavage- way,' the 1 quartering-way,' and the ' tough-way.' These terms are employed somewhat differently in different granite districts.
58 GEOLOGY OF BUILDING STONES
and care has to be exercised, when visiting the quarries, to avoid confusion due to thinking that the quarryman means what you mean by the same expression. Thus, while the rift is always the direction of most easy cleavage, it may be coincident with what in one quarry is called the ' cleavage- way,' and in another with what they call the * quartering- way.' Careful observation in the quarry will usually enable one to distinguish the three directions even in detached blocks while in their rough state ; but it is sometimes by no means easy, and requires considerable experience.
Dykes, Veins, and Allied Structures. — The regular homo- geneous character of the granite in many quarries is occasionally interrupted by portions which differ from the main mass, both in texture and composition ; and where they are numerous or developed upon a large scale they may interfere seriously with the processes of quarrying.
The most striking of these variations are the dykes, sheets of igneous rock which traverse the granite in any direction, either as simple wall-like masses and tabular layers or as branching and anastomosing intrusions. In all cases the dykes are younger than the granite in which they lie. As regards composition, dykes may be very similar to the surrounding rock, differing merely in the average size of grain ; or they may be more siliceous (acid) than the granite, or less siliceous (basic).
The most common form of siliceous dyke is the rock known as pegmatite, in which it is customary to find the same type of felspar and mica as in the surrounding rock ; but the individual crystals are much coarser, and often very perfectly formed. To anyone familiar only with the granite of commerce, the size attained by the crystals in some pegmatites would excite no little wonder; for it is not uncommon to find mica in tabular masses from i foot to 4 or 5 feet in diameter, or felspars many feet in length and of proportionate girth. One crystal of felspar in the
IGNEOUS ROCKS 59
University Collection at Christiania is about 7 feet long. In some pegmatite veins or dykes a parallel intergrowth of the felspar and quartz (pegmatitic structure) is a marked feature. In addition to the ordinary minerals of the surrounding granite, other and rarer species are frequently developed, and may assume commercial importance.
Veins or dykes of granitic rock in which paucity of mica is associated with comparative fineness of grain are known as aplite. The majority of veins or dykes of pegmatite and aplite are narrow, from a few inches to a foot or two > but they may occasionally be as much as 20 to 30 feet in width, and where the pegmatite is as a marginal feature of the granite, it may assume great dimensions, reaching to hundreds of yards. Similar variations of texture and composition are found in the mids* of some granites in the form of spheroidal or irregular masses.
It has been indicated above that dykes and veins inter- rupt the regular working of the quarry, and the material therefrom is usually waste ; but in some districts they are put to use in the form of copings for garden walls, and in the formation of rockeries and stone borders. Blocks of pegmatite have been brought to the London district from Scandinavia for rockeries, in which, by reason of their bold * figure,' they make an attractive substitute for the familiar and sordid-looking clinker or burnt brick.
It should be pointed out that the characteristics of pegmatite — namely, coarseness of crystallization and a tendency to greater acidity than the parent igneous rock — are not limited to the granites, but may be observed in relation to other more basic rocks — syenite, gabbro, etc.
In all cases — whether their mode of occurrence be as broad dykes, narrow ramifying veins, druses or geodes, coarse isolated patches, or in the form of marginal tracts— the pegmatites may be regarded, according to Harker, as representing the residual 4 mother liquor ' at the end of the.
60 GEOLOGY OF BUILDING STONES
process of crystallization — a view which readily accounts for the peculiarities of structure and composition of this rock variation.
The more basic dark veins and dykes that are sometimes found penetrating granite masses have usually no genetic relationship with the rock they traverse, but are of a newer and later phase of activity. The small ' knots ' or rounded masses, rich in dark minerals, mica, hornblende, and the like, are usually early basic segregations or secretions formed from the liquid crystallizing magma. If the knots are small they are disregarded in quarrying and dressing the stone, but when they are large they are treated as waste. They are to be seen in blocks of some granites
Later Acid Intrusion ••Elvan"
V
Sedimentary Rock /'
Dyke and Sill
FIG. 5.— GRANITE AND ELVAN DYKES AND VEINS OF BASIC ROCK.
when set up in buildings, but they should not be permitted to appear in stones that have to take any moulding or carving. Dark included masses, often similar in appear- ance to the basic segregations, are portions of the sur- rounding rock caught up by the granite during its intrusion. Dykes and veins receive various names from the quarry- men, which differ in different districts. Practically all the veins in the Cornish area are called 'elvans' (see p. 94) whatever their petrological characters ; some are diabases and similar rocks, but the majority are micro-granites and quartz-felsites or quartz-porphyries. Sometimes the.
IGNEOUS ROCKS 61
veins are called ' horses,' white or black as the case may be.
In the immediate neighbourhood of dykes the granite is frequently unfit for use, and has to be left standing or removed as waste.
Colour. — Except in the case of engineering works, the colour, or rather the appearance, of granite is the quality which perhaps more often than any other determines the final selection of the stone. Notwithstanding this fact, the variable nature of the causes which produce the colour effect and the subtle nuances of tint and texture in the rock make it wellnigh impossible to convey in words the innumerable different impressions that the mind readily assimilates through the eye.
It is easy to speak of dark red, red, pink, flesh-colour, greenish, and grey granites, and to convey thereby a general idea of the colour ; but when it comes to dis- tinguishing between the numerous pinks and the infinite variety of greys, language fails, and nothing can take the place of a personal acquaintance with the stones themselves.
The dominant colour of granite is nearly always deter- mined by that of the felspars (q>v.), but this is modified by the colour and disposition of the other minerals, the quartz, mica, or hornblende, and still further by the degree and kind of alteration that has taken place in these minerals by the production of secondary minerals.
The appearance of granite is influenced by more than the mere colour of the fresh stone ; it is compounded of the several effects produced by the state of aggregation of the minerals — felspars of one or more kinds, porphyritic or non-porphyritic, large or small ; quartz in small individuals evenly distributed, or in large clots of grains — the presence or absence of flow structure or of basic knots at frequent inter- vals; the presence or absence of bright cleavage faces in the
62 GEOLOGY OF BUILDING STONES
felspar, which may glitter and shine by the light they reflect j and finally by the nature of the surface given to the stones by polishing, axing, picking, etc. Polishing and rubbing invariably produce a darker colour effect than fine-axing.
Chemical Composition ; Mineral Composition. — The
chemical analysis of a granite informs us of the composi- tion of the aggregate of minerals which compose the rock; if, therefore, we are familiar with the average composition of the essential minerals, we may foretell the rock analysis with sufficient accuracy for most practical purposes, if the minerals can be recognized and their proportionate repre- sentation in the stone can be estimated. The necessary determinations can be made by observation and simple measurements on thin micro-sections of the stone, and on a polished face or clean fracture. For this reason the two characteristics — mineral and chemical composition — will be considered together.
Given a stone with granitic structure, be it coarse or fine in texture, it may be regarded as a granite if its silica content does not fall below a minimum of 66 per cent. This is the normal minimum for the silica content ; it is true that some abnormal rocks with a slightly lower percentage would be classed with granites by some petrologists, but they would not be so recognized by others.
A rather curious error is not at all infrequently made by many who read analyses prepared for trade purposes ; it is to regard the silica expressed in the analyst's report as representing the crystallized silica, quartz (this applies to all analyses of stone). This, of course, is by no means true. The total silica content embraces not only the free quartz, but that which is combined with other elements in the other minerals.
The analyses of granitic minerals in Table V. will make .this clear :
IGNEOUS ROCKS TABLE V.
|
*1. |
2. |
3. |
4. |
|
|
SiO2 (Silica) A1203 ... . Fe203 ... . FeO |
67-99 19-27 0-28 |
47*95 30-26 2'43 'VIO |
35'55 , 17-08 23-70 3'rr |
54^9 I-50 5-06 7',l6 |
|
MnO MgO ... . CaO |
O'O2 O"7£ |
°'94 0*08 |
O JJ i '95 3-07 0*61 |
/ V 16-01 1 2 -08 |
|
Na20 K9O ... |
; '5 6-23 O'OC |
2'OO 10*25 |
o'35 Q'AC |
0-37 0-38 |
|
H«O .. |
J *O O'QO |
2-8=; |
V TO d"?O |
2*72 |
|
- "D |
||||
|
Total |
99-03 |
10076 |
99-6I |
IOQ'47 |
* i, Felspar, Anorthoclase (or AbAn) ; Minnesota, U.S.A. 2, A White Mica, Muscovite ; Fichtelgebirge. 3, A Black Mica, Lepidomelane ; Ireland. 4, A Hornblende (Green) ; Schwarzwald.
These analyses will serve to illustrate how widely, not only the silica, but almost all the other simple molecules, are distributed among the main constituent minerals.
There is a sufficiently wide range among the granitic felspars to permit of a simple classification of these rocks into alkali-granites and lime-alkali-granite. In the former group, depending upon the relative predominance of potash (or soda-bearing) felspars, we have potash- granites (with orthoclase, microcline, or perthite) or soda- granites (with albite, anorthoclase, or soda-orthoclase) ; in the latter group a certain amount of lime-bearing felspar (oligoclase) is present along with, but not in excess of, the alkali felspars. These varieties of granite pass by insensible gradations one into another, and by the gradual increase in the proportion of lime-bearing felspars they shade off into rocks which are near to diorites (p. 85);
64 GEOLOGY OF BUILDING STONES
the intermediate stages are the grano-diorites or quartz- monzonites of some authors. In order to have some agreement as to the limits of these arbitrary subdivisions, it has been proposed to class them as follows :
Most alkaline ... Alkali-granites, in which more than two-thirds of the felspars are alkali-bearing.
Medium alkaline ... Intermediate, 'adamellite' type, in which less than two-thirds and more than one-third of the felspars are alkali-bearing.
Least alkaline ... Grano-diorites, in which less than one-third of the felspars are alkali-bearing.
If it could be clearly proved that the granites with lime-bearing felspars were actually the less durable as employed in constructional work, it would be worth while to emphasize the importance of this classification. In the present state of knowledge, however, it does not appear to possess much practical significance.
Another mode of distinguishing the varieties of granite is to group them according to the dominant coloured or ferro-magnesian mineral. Thus, if dark mica alone is present the rock is a biotite-granite — this is a very common type; a Muscovite- biotite- granite contains both light and dark mica ; a Muscovite- granite is almost devoid of dark mica ; if hornblende is associated with dark mica the rock is a hornblendt-biotite-granite ; with hornblende alone it is a hornblende-gran.te. Then there are the less common varieties tourmaline-granite, augite-granite, riebeckite- granite, and so on.
The student should bear in mind that this nomenclature has been employed in a very rough-and-ready way by writers.
For the purpose of giving a general idea of the composi- tion of granites from different parts of the world, a few illustrative examples will now be considered :
IGNEOUS ROCKS 65
i. Red Granite: Percentage of Minerals estimated by Measurements.
|
1 |
2. |
3. |
4. |
|
|
Felspars |
65'3o |
55-9I |
49-92 to 70-83 |
68 |
|
Quartz ... |
28-65 |
35-C6 |
23-04 to 41 08 |
27 |
|
Mica (biotite) ... |
5'55 |
8'43 |
4-72 to 11-29 |
5 |
2. Grey Granite.
|
5. |
6. |
Average. |
7. |
|||
|
Felspars ... |
58-86 |
Felspars |
55-801069-51 |
60 '02 < |
Felspars |
50'05 |
|
Quartz Hornblende |
33-88 7-26 |
Quartz Kiebeckite and Jigirite |
22 '06 to 33'7I 7'47 to 11-10 |
30-60 9'37 |
Quartz Mica ... |
37-55 12-40 |
A more detailed estimate of the minerals in Delank granite obtained by Rutley with Delesse's method yielded the following result :
Felspar, orthoclase ... ... ... ... ... ... 30
Felspar, plagioclase ... ... ... ... ... 6
Mica, biotite 7
Mica, muscovite ... n
Quartz 46
100
Compare with this the result of measurements on a fine-grained grey granite from Milford, New Hampshire :
Felspar, soda-lime (oligoclase) 34'°3
Felspar, potash (microclinc, 14*15 ; orthoclase, 15-57) 29-72
Mica (biotite) 8-58
Quartz 27-09
Magnetite 0-25
Minor accessories ... ••' o'33
lOO'OO
5
66
GEOLOGY OF BUILDING STONES
It may be of interest to compare the chemical analyses with the mineral percentages in the foregoing table ; the numbers at the head of the columns in Table VI. corre- spond with those used on p. 65.
TABLE VI. Chemical Analyses of Some Red Granites.
|
1. |
2 and 3. |
4. |
|||
|
Si02 A1203 . |
71-44 1472 |
72-02 H*43 |
76-07 12*67 |
77-08 12-54 |
68-55 16-21 |
|
Fe203 . |
2-39 |
1-25 |
2'OO |
2-26 |
|
|
FeO |
0*46 |
0-89 |
— |
o'95 |
— |
|
CaO |
— |
1-8 |
0-85 |
075 |
2-40 |
|
MgO |
0*96 |
trace |
O'lO |
O'OI |
1-04 |
|
Na20 |
7-66 |
5-85 |
3'37 |
3' '4 |
4-08 |
|
K20 |
0-89 |
5HI |
4-71 |
4'9;> |
4-14 |
|
TiO2, ZrO2, etc. |
078 |
||||
|
H2CT (ignition) |
o'6i |
o"35 |
— |
— |
— |
|
99-91 |
0-71 |
99-80 |
99-96 |
99' 13 |
Chemical Analyses of Some Grey Granites.
|
5. |
6. |
7. |
8- |
|
|
SiO2 |
77"6l |
7TQ^ |
72-84 |
72'OC |
|
TiO9 . |
O"2^ |
0-18 |
||
|
A1203 %&... ::: CaO ... |
\j ^.^ 11-94 o'55 0-87 O"2T |
12-29 2-91 1*55 O"2 T |
16*25 0*14 1-49 I" IO |
I5-83 0-39 I-50 I ' 14 |
|
MgO... |
*-* JL trace |
w Ox 0*04 |
O'CC |
i ^4 O'CT |
|
Na2O K2O |
3-80 4*08 |
4-66 4*6'? |
** DD 2-25 5"IQ |
9 2-65 4"7Q |
|
MnO H2O (ignition) |
trace 0-23 |
trace 0-41 |
0*63 |
t /V 0*64 |
|
100-54 |
100-91 |
IOQ'44 |
99-50 |
IGNEOUS ROCKS 67
Brief descriptions of these granites are appended :
1. Redstone Granite, Conway, New Hampshire. — Coarse, pinkish, mottled, with amethystine grey quartz and spotted with black. A biotite granite passing into a biotite-hornblende granite. The pinkish felspar is ortho- clase minutely intergro\vn with soda-lime felspar (oligo- clase - albite) ; there is also a little oligoclase - albite separately crystallized.
Compressive strength, 22,370 pounds per square inch.
2. Milford Granite, Massachusetts. — A biotite granite of medium to coarse texture, not porphyritic, even-grained. The felspars are pink to cream-coloured. Orthoclase and microcline minutely intergrown with soda-lime felspar, a subordinate yellowish to white albite to oligoclase-albite, slightly kaolinized ; faint blue quartz somewhat fractured. No. 3, p. 65, shows the range of estimated mineral per- centages in the more gneissose stone from the same locality. Nos. 2 and 3, Table VI., show the analyses from three neighbouring quarries in the district.
Compressive strength, 20,000 to 29,200 pounds per square inch.
4. Shap, Cumberland (p. 70).
5. Rockport Grey Granite, Pigeon Hill Quany, Massa- chusetts.— A hornblende granite, of medium to coarse texture, even-grained. Colour, medium grey, sometimes with greenish or bluish tinge, with black spots. Felspars, grey orthoclase with occasional microcline, with minute intergrowths of albite to oligoclase-albite; very little of the latter kind is separately crystallized. The quartz is smoky, and contains many cavities and bubbles. The hornblende is dark, and a very little black mica is present.
Compressive strength (three tests), 20,716, 20,522, 17,772 pounds per square inch.
6. Quincy, Massachusetts. — A riebeckite-aegirite-granite. Colour, a medium grey with greenish, bluish, or purplish
68 GEOLOGY OF BUILDING STONES
tinge, passing to a very dark bluish-grey, with black spots. The trade names for these shades are, ' medium,' ' dark,' and ' extra dark.' Texture even, medium to coarse. Felspars, orthoclase, with minute intergrowths of albite to oligoclase-albite. The several colour shades in the grey felspar are produced by minute inclusions of epidote (green\ hornblende (dark brown), and riebeckite (bluish). Some of the lime-soda felspar is crystallized separately. Riebeckite, looking blue black, and aegirite, greenish-black, take the place of the usual mica, etc. The quartz is smoky and bluish. The stone takes a high polish.
7. Penryn, Cornwall (see Appendix A). — Compressive strength, 19,450 pounds per square inch, 1,250 tons per square foot.
8. Carnsew, Cornwall (see Appendix A). — Compressive strength, 22,336 pounds per square inch, 1,436 tons per square foot.
Physical Characters of Granites. — In a discussion of the physical properties of any rock group, it must be constantly borne in mind that we are dealing with materials which show a remarkable amount of variability. Take the granites, for instance ; even if we confine our attention strictly to those rocks which are granites in the petrological sense, how diverse is the material in its obvious structural conformation alone, to say nothing of those obscure differ- ences of rift and fracture which make one granite easy, another difficult, and another impossible, to work with profit.
In this and subsequent chapters dealing with other stones, the remarks upon the physical characters are there- fore to be interpreted as generalizations only.
Specific gravity 2'6 to 2-8.
Weight per cubic foot ... 160 to 200 pounds.
Resistance to pressure ... Usually from 1,000 to 2,200 tons per
square foot, but samples may fall as low as 800, or rise as high as 2,740.
IGNEOUS ROCKS 69
DISTRIBUTION OF GRANITES.
England. — All the granites now worked in the West of England are grey. Reddish granite does occur on a small scale in the St. Austell district, but it is quite unimportant. The red granite of Trowlesworthy Tor, which has a pleas- ing but rather peculiar colour — not pink— is not now quarried.
The constituents of Cornish granite are orthoclase and micro-perthite, often large and porphyritic, quartz, and a fair amount of plagioclase which is largely albite. Both light and dark mica are present, and tourmaline is a common accessory mineral. The large porphyritic crystals are not present in all the stones. They often exhibit zonally-arranged inclusions of dark mica, and not infrequently twinned crystals appear in section as white crosses. The large crystals sometimes lie with a fairly regular orientation, but this is not strongly marked as a rule. On the borders of the Penryn mass, in the Bodmin mass, and elsewhere, a granite of much finer grain appears ; it is rarely quarried, however, beyond the upper stained portions, though it appears to be a good stone.
One of the standard textbooks on building materials refers to Cornish granite in rather disparaging terms; it is hardly necessary to-day to point out how ridiculous it is to compare the locally kaolinized granite — here the work of deep-seated agencies — with the granite in its normal condition. Nor is it any longer correct to refer to the Cornish granite as ' moorstone,' a name which was quite applicable in the early days of the industry prior to the formation of deep quarries. The earlier stones were obtained from the large loose blocks which in those days were, and are still in many places, strewn over the moors. (For further details as to granite quarries see Appendix A.)
In the Midland Counties true granite is found only in
70 GEOLOGY OF BUILDING STONES
Worcestershire and Leicestershire, and in neither district is the granite quite of the normal type. In Worcestershire a gneissose granite occurs at North Hill, and a hornblende granite appears north of Wych, Great Malvern. The Mount Sorrel granite in Leicestershire is richer in plagio- clase than ordinary granite, orthoclase is subordinate, and hornblende occurs in addition to biotite. The rock has in parts the character of a grano-diorite. The stone varies a good deal in colour ; a light kind is rather dark grey, with paler roundish felspar crystals scattered rather sparingly in a smudgy, indefinite-looking ground mass containing grains of bluish-grey quartz and patches of dark hornblende. Another darker variety has the same structure, but the ground is dark reddish-brown, and the porphyritic felspars are paler red. Yet another variety is very fine-grained, without any obvious structure, and of a rather dead brown appearance. Some varieties are greener than others ; it is not much used for buildings, although it takes a good polish, but it is largely quarried for setts and macadam.
Passing northward, no more granite appears until Shap Fell is reached in Westmorland, where the rock crops out as a compact mass some two and a half miles by two miles. This rock has a very striking appearance on account of its distinctive colour, a rather brownish-red, with large porphyritic crystals of flesh-coloured orthoclase ; the finer-grained part of the stone occurs in several shades from grey'to warm brownish- red (Light and Dark Shap). The porphyritic crystals frequently exhibit a roughly parallel and meandering flow arrangement. The rock is a biotite granite with pkgioclase and orthoclase ; it is of interest to note that, contrary to the usual rule, the quartz is enveloped by the orthoclase.
Shap granite is obtainable in large blocks, and has been much employed as a decoration stone ; it polishes very
IGNEOUS ROCKS
BASIC DYKES
IGNEOUS ROCKS. ENGLAND AND WALES.
Granite.
Other Igneous rocks.
LAKE DISTRICT VOLCANIC ROCKS
&SHAP GRANITE
VOLCANICt \INJRUSIVE ROCKS
DOLERITES
OF DERBYSHIRE
IGNEOUS ROCKS OF LEICESTERSHIRE* WARWICKSHIRE
Clee Hills
Builth
Malrern Hills
' BASIC & ACID
VOLGA NIC t INTRUSIVE ROCKS
^
* GRANITES OF CORNWALL & DEVON WITH ELVANS t BASIC VOLCANIC ROCKS DYKES
, SERPENTINE & GABBPO
\-
MAP I. — IGNEOUS ROCKS OF ENGLAND AND WALES,
72 GEOLOGY OF BUILDING STONES
well. It is also used in engineering work, dressings, macadam, and for concrete paving slabs and artificial stone. It has been used in all parts of England, in America, and other places abroad. It may be seen in the columns at St. Pancras Station, the posts round the enclosure at the western entrance to St. Paul's Cathedral, Temple Bar Memorial, and very many shop-fronts ; also in the graving-docks, Southampton, the harbour works, Heysham, North- Eastern Railway, and the viaduct over the Severn at Shrewsbury.
A large mass of granite occurs at Eskdale, in Cumber- land; it is irregular in shape, about thirty -five square miles in area, and extends from Wastdale and Boot, near Eskdale, southwards by Muncaster and Dayockwater nearly to Bootle. There are several varieties, but most of it is coarse-grained, grey, and porphyritic ; the felspar is mainly perthite with orthoclase and oligoclase, abundant quartz, and both light and dark mica. Some of this rock is granophyric, and the quartz in some cases has crystallized early, as in the Shap granite. It has not been much worked.
Scotland. — The principal occurrences of granite in Scotland are those of Kirkcudbrightshire, Aberdeenshire, and Kincardineshire ; to these must be added the granites of the Highlands and the Western Isles, most of which are too inaccessible to be of value for commercial purposes.
The Kincardineshire- Aberdeenshire granites, embracing the well-known rocks of Peterhead and Aberdeen, cover a large area, and present many points of great interest to the petrologist ; but over a great part of the region much of the stone is unsuited for constructional purposes. Most of these worked rocks are biotite granites, with oligoclase in varying proportions along with orthoclase or microcline ; on the whole they are less like grano-diorites, and more siliceous, than those in the Galloway district.
IGNEOUS ROCKS 73
In the Aberdeen district, where granite is essentially a material for construction, preference is given to certain kinds of the stone for specific purposes. For domestic architecture, ashlar stone is generally specified to be from Rubislaw, Kemnay, Sclattie, Oldtown, Toms Forest, and Tillyfourie; while Kemnay is specified for lintels, sills, and strong courses, or wherever finely dressed parts are required for emphasis. This stone has a very bright appearance when finely tooled. The darker grey granites, such as Rubislaw, are very effective in rough ashlar. Of the coloured granites, Corrennie is much used. Kemnay, Tilly- fourie, and Toms Forest, are used for engineering work, and when dressed are only distinguished, if at all, with difficulty.
In the Peterhead district the characteristic stone is a biotite granite, with microcline and some plagioclase, and smoky or clear quartz. The texture is moderately coarse and not porphyritic ; the common colour is a dark flesh tint; the grains of the constituent minerals have ill- defined boundaries, but stand out clearly. The stone from the Cairngall quarry is of a very beautiful cool grey colour, with small irregular-shaped white felspars sprinkled in the grey ground ; good examples of the polished stone may be seen in the wall decoration of the hall of the Museum of Practical Geology, London. Large blocks were used in the Prince Consort's sarcophagus ; eight columns in St. George's Hall, Liverpool (shafts 18 feet high) ; round the fountain at Trafalgar Square ; the lintel over the door of the Duke of York's Monument ; and the pedestal of the Duke of Wellington's statue at the Royal Exchange, Glasgow.
The red Peterhead stone is almost exclusively used for polished w^ork. It has been employed in the Duke of York's Column (1830) ; the columns of St. George's Hall, Liverpool; in the Fishmongers' Hall; and many monu- ments and shop-fronts.
74 GEOLOGY OF BUILDING STONES
The granites quarried in Kincardineshire are similar to those in the Aberdeen district ; the Cove quarries are the most active. The stone is dark grey, and of medium- sized grain ; the felspars are small and white, and the quartz is in rather larger grain ; a roughly-marked foliation is sometimes apparent. The stone is mostly used for roads. Nigg Quarry, a little north of Cove, yields a finer- grained, darker grey stone. From the Hill of Fare quarries, north of Banchory, a fine-grained dark red stone, finer- grained than Peterhead, Birsemore, or Corrennie, was obtained and employed in a memorial to Queen Victoria, Windsor Castle, and in the polished part of the Byker Bank, Newcastle. In an axed state it may be seen in the single- span bridge at Kelvin Park, Glasgow.
The granite of Kirkcudbrightshire and the adjoining counties appears protruding through the sedimentary rocks in three large and a number of small quarries. The large protrusions are (i) Criffel and Dalbeattie ; (2) the Cairnsmore of Fleet ; and (3) the Loch Dee mass. All the principal quarries are situated in the first of these. The stone is usually a bright clean grey ; in mineralogical composition it is subject to some variation, but generally it is much richer in lime-bearing felspar than the Aberdeen rocks. The Criffel and Dalbeattie stone is moderately fine-grained, and consists of a white oligoclase felspar in a finer ground mass of quartz and orthoclase, together with dark mica or hornblende, or both — sometimes with augite. The rock is evidently in parts much nearer a quartz-diorite than a normal granite.
For further details as to quarries of Aberdeen, Peter- head, and Kirkcudbrightshire granite, see Appendix A.
Granites of the same period as those worked about Aberdeen occur in Nairn, Elgin, and Inverness, Ben Rinnes, Grantown, and farther north at Ben Loyal, where there are considerable quarries ; in Strath Halladale, Lairg,
IGNEOUS ROCKS 75
and large masses on the borders of Sutherland and Caith- ness, and in the Ord of Caithness.
The granites of the South-West Highlands are richer in plagioclase than those of Aberdeen, and more closely resemble those of Kirkcudbrightshire and Wigtonshire. The larger masses are those of Rannoch Moor, Ben Cruachan, and Loch Etive; here are the large Bonawe quarries at Taynuilt, conveniently situated near the loch. This is a strong, moderately fine-grained stone, greyish- blue or greenish -grey in colour, a shade darker than Kemnay, with much plagioclase, little quartz, biotite, a little green hornblende, and some augite. It is mainly used for setts and macadam. In the same county are the quarries at Craigmore, near Taynuilt, Ardshiel, and Blackwater Dam at Ballachulish. This is moderately fine-grained, rough when axed, but with a lively appear- ance when polished ; it is darker than Bonawe stone ; the felspars are pale salmon or buff, evenly mixed with a white variety (not porphyritic), and a considerable amount of mica.
Some of the granites of Loch Etive are pale brown or brown in tone, of coarse to medium-coarse texture ; that from Bars has both white and pinkish felspars developed porphyritically ; the High Rock stone is less regular in appearance.
A red granite is found in Argyllshire, with bright red felspars in clear white quartz ; both minerals form irregu- lar grains.
An extremely coarse rock (pegmatite) from Portsoy, Banff, consists of irregular masses of pink felspar along with great masses of quartz full of strong flakes of mica.
In the Western Isles there are several granites, many of which are much younger than those of the mainland. Most of the granites of Arran, that of St. Kilda and Beinn an Dubhaich in Skye, are biotite granites. Those of the
76 GEOLOGY OF BUILDING STONES
Red Hills in Skye, and in Mull and Rum, have a lower silica percentage, while hornblende and augite take the place of mica. The Ross of Mull granite is warm red in colour, with red orthoclase and some white plagioclase. It is used in the piers of Blackfriars Bridge.
The fine-grained drab-coloured rock of Ailsa Crag may be mentioned here as an example of a riebeckite-micro- granite.
The rock largely quarried at Furnace, Lochfyneside, though known as a granite, is a quartz-porphyry; it is used for setts, curbs, and crushed stone. In appearance it is much like some of the white, speckled-grey Cornish elvans.
Ireland. — Times without number it has been pointed out — quite truly — that there is abundance of good granite in Ireland. The principal quarrying district embraces the irregular granite mass near Newry, Co. Down, the smaller mass between Carlingford Lough and Dundale, and that forming the Mourne Mountains.
The Newry granite is much older than the other two ; it consists of quartz, orthoclase, and an alkali-lime felspar, with biotite and hornblende. The colour is generally greenish-grey or grey-blue. Setts constitute the main product from this district, but large blocks may be ob- tained, and the stone has been widely employed as a building and polished monumental stone.
The next important granite district is in the South-East of Ireland, where a large elongated mass stretches from the coast at Kingstown in a south-westerly direction for about fifty-six miles, mainly in the counties of Carlow and Wicklow. The bulk of this rock consists of quartz, potash felspar, often microcline, subordinate plagioclase (albite and oligoclase), and both light and dark mica. Except in local variations, its texture is moderately coarse. The principal quarries in this granite are Parnells (Arklow), Glencree, Ballyknocken, and Ballybrew. In Dublin County
IGNEOUS ROCKS 77
there are numerous small quarries producing granites of medium texture and various shades of grey: Glencullen, Barnacullia, Ballyedmanduff, and Balally. Some of the local granite used in Dublin City has been very badly selected. Kingstown Harbour was built with stone from Dalkey quarries, and it is used in the churches of St. Paul and St. Wefburgh, Dublin. Quarries on Killiney Hill supplied stone for the Thames Embankment, the pier and harbour, Kingstown, and many large buildings in Dublin. (See Appendix A for Irish Granite Quarries.)
Granites (silvery grey and red) have been quarried in Donegal, at Fanad, Mulroy, and Fairy Castle quarries. Good red granite is found in Galway ; some of it is rather like a fine-grained Peterhead in general tone, but with a tendency to have large pink porphyritic felspars, in a ground of pink and cream-coloured felspars, abundant quartz, and a moderate amount of mica ; other varieties ire rather coarser, and some blue-grey stone also occurs. The principal quarries are at Shantallow.
In the Isle of Man granite is worked in the Dhoon quarries near Ramsay, and in the Foxdale quarries ; these stones are muscovite-biotite granites with much oligoclase, some microcline, and abundant quartz ; it is of medium texture and light grey colour. The Dhoon stone is more porphyritic than the other.
A biotite granite is quarried at St. Brelades, Jersey.
In Wales, although there is a great development of igneous rock, there is little granite ; the largest outcrop is in the Lleyn Peninsula, Carnarvonshire. This is five miles long from north to south, and about two miles in breadth. The rock is a biotite granite with altered orthoclase and some oligoclase.
Some Foreign Granites. — Of the foreign granites, those which exercise the greatest influence on the British trade
78 GEOLOGY OF BUILDING STONES
come from Scandinavia. Large quantities of stone are shipped for curbs, setts, ornamental, engineering and architectural work.
For ornamental purposes a good deal comes to Scotland to be polished.
Sweden. — On the west coast the Bohuslan district yields many grades of stone, from light grey to pale red in colour, and fine to medium-coarse in grain. There are quarries at Lysekil (coarse reddish), Idefjord, Malmon, and many other places.
In the Halland district much of the stone is gneissose, and variable in colour ; it is largely used for curbs. The * Varberg granite ' is a grey, red, or dark green rock, used for polished work; it does not weather well. From the western side come the red granites of Vanevik and Virbo in Smaland ; these have a medium to coarse grain with the quartz ; they are known as ' Red Swedish ' in Britain and America. The brilliant crimson Uthammar granite is called ' Bon Accord Red ' (some of the Swedish Bon Accord is an olivine-gabbro). From Jungfrun Island a red stone known as ' Virgo granite ' is obtained. Red and grey granites are worked near Norrtalje, for use in the Stockholm district. The ' Stockholm granite ' much re- sembles the Aberdeen grey. From the Graversfors district the ' Swedish Rose ' granite is obtained ; this has dark red felspar with deep blue and purple quartz. Other varieties are coarse-grained with brown felspar and blue quartz. The granites and gneiss of Blekinje — fine-grained, red, and grey — are mainly used for curbs and setts, and sent to Germany.
Norway. — Great quantities of curbs and setts are ob- tained from the Norwegian granite, but the principal architectural stone comes from the Liholt quarries in the Idefjord. This is an excellent fresh grey stone of medium grain ; it has been much employed in large buildings in
IGNEOUS ROCKS 79
Christiania — e.g., the New Theatre, Freemasons' Building, Sparebank; and in the Royal Liver Building, Liverpool, and many others in this country ; it is known as ' Grey Royal.' A similar stone from this district is the ' Imperial Grey.'
The United States of A merica.— Granites are worked in many of the Eastern and Western States ; the stone is often gneissose in structure. In Maine the proximity to deep water, and the freedom from ' stripping ' or over- burden, produce favourable conditions, similar to those which obtain in Scandinavia. The most extensive quarries are those of Vinalhaven or Fox Island, a coarse stone, grey to pink in colour. The Augusta and Halloweli quarries yield a light grey stone used for the large figures in the Pilgrim Monument, Plymouth, Mass. The Mount Desert and Crotch Island granites are light to dark grey and dull pink. Red Beach is pink to red ; Otter Creek, medium-grained, dull red ; Mount Waldo, light grey ; East Blue Hill, grey to pinkish, and sometimes porphyritic.
The Quincy, Rockport, and Gloucester granites of Massachusetts are rather coarse granites and gneisses, often hornblendic, and dark blue-grey to pink in colour.
In Wisconsin the principal granites are Montello, bright red and grey-blue ; Waushara, pale pink and red ; Amberly, fine grey to coarse red ; Warsaw, grey, brown, and brilliant red ; Granite City, reddish-grey and red ; Waupaca, coarse, with brown, red, or pink felspar in a greenish to black ground.
According to Merrill, a very beautiful coarse red granite is found near Lyme in Connecticut ; the red felspars have the unusual property of being clear and transparent ; it is used in Newport and Rhode Island.
Granite is found in California, Georgia, Maryland, Minnesota, Utah Territory, Virginia (fine grey), Washing- ton, New Hampshire, and several other States.
In Canada there is plenty of good granite, but it is not
8o GEOLOGY OF BUILDING STONES
yet exploited to any great extent. In Eastern Quebec, near Stanstead and Staynerville, the pale grey granite is quarried for roads and for building. Red granite has been worked at Kingston, Ontario, and at Barrow Island, Chatham, Wentreath, Granville. The so-called ' Bay of Fundy ' granite is a medium-grained red hornblendic rock from St. George, New Brunswick. In Halifax Count}', Nova Scotia, a grey granite is quarried ; a grey stone is found at Burard Inlet, British Columbia.
France. — In France granite occurs in Normandy, Brit- tany, the Vosges, and in the Central Plateau.
The following are a few selected types : Laber granite, near Brest, occurs in large blocks ; its colour is grey, pink, and blue, fine-grained, with large porphyritic felspars ; used in sea-walls at Bordeaux, Cherbourg, Havre, and Thames Embankment, and in the pedestal of the Luxor Obelisk, Paris. Alencon granite, fine to medium grain ; pale yellow to grey-blue ; used in the principal buildings of Alen9on. lies Chausey granite, near Granville ; fine-grained, hard, bluish-grey. Combourg, medium grain, bluish. Pontivy, moderately hard, yellowish-grey. Vosges granite, Geradmer ; porphyritic, variable colour. Remiremont, very hard, bluish-grey. The granites of Chamonix and Epierre are talcose or protogine granites, hard and white, spotted with green. The so-called Kersanton granite, from near Brest, is a dyke rock consisting mainly of plagioclase felspar, dark mica, and quartz ; it is not a true granite, but a variety of dyke rock called Kersantite. It is largely quarried and very durable ; it is fine-grained, and light bluish or greenish-grey, which darkens on exposure; the sixteenth-century church of Vannes is built of it, and it is used for architectural and engineering work.
Austria. — Most of the granites are grey or bluish-grey, fine and coarse ; they are quarried in the south at Meiszau and Gmund in the Tyrol ; in Steiermark ; Budweis,
IGNEOUS ROCKS 81
Carlsbad, Przibram, Petersburg - Jechnitz (Pilsener granite), and elsewhere in Bohemia ; Setzdorf and Friede- burg in Silesia ; in the north at Mauthausen (much used in Vienna), Dornach, Neuhaus, etc.
Germany. — The granites of Saxony are worked in the districts of Meissen, Oberlausitz, and in the Erzgebirge and Fichtelgebirge. Those of the first-named district are reddish and rather coarse ; those of the second are pale blue and of medium grain ; both grey and red granites of fine to medium grain are found in the third, and there are many quarries.
The Silesian granites are mostly pale grey and of medium grain ; there are quarries at Striegau, Strehlen, Gorlitz, and Oberstreit. In the Odenwald a dull red medium-grained granite is worked at Fahrenboch and Felsberg. In the Bavarian Mountains the Blauberg granite is quarried at several places. Granite is worked in the Black Forest at Oberkirch, Triberg, Waldshut, Gernsbach, etc. ; and in Thuringia a red stone comes from Brotterode, Ruhla, Mehlis, etc.
Among the granites used in ancient times that from between Assuan and the first Nile cataract — the red hornblende-biotite-granite of Syene — is one of the best known.
In India there are many occurrences of granite and- gneiss, some of which were used in ancient temples, and carved with extraordinary patience; sometimes the whole edifice was cut out of a single rock mass. More recently they have been employed for engineering, as in the case of the Raichur granite for railway bridges and works of the Bombay Port Trust.
CHAPTER IV OTHER IGNEOUS ROCKS
As a material for construction, whether in architecture or in engineering work, granite far exceeds in importance all other igneous rocks. This is due in part to the in- herent good qualities of the stone, its strength, its adapta- bility to various styles of dressing, its satisfactory colour and appearance, and its susceptibility to polish ; but its popularity is also accounted for by the fact that its occur- rence is not limited to a few isolated tracts — it appears in tolerable abundance in every quarter of the globe.
Many of the other igneous rocks are less widely dis- tributed, and though they may be familiar building stones in certain localities, their qualities are not generally so well known to architects.
In the following brief descriptions no attempt will be made to include all the igneous rocks that at one spot or another have been employed in building ; a selection has been made to embrace some of the more prominent types, and these are presented according to no strictly petrological order, but in groups according to their mode of origin —namely, (i) Plutonic, (2) Hypabyssal, (3) Volcanic.
PLUTONIC — HOLOCRYSTALLINE ROCKS.
Syenite. — The syenites are rocks of granitic texture, more often coarse to medium-grained than fine-grained. The prevalent minerals are alkali felspars (orthoclase),
82
OTHER IGNEOUS ROCKS 83
etc., or the felspathoids may more or less completely take their place, and in either case there may be in addi- tion a little lime-soda felspar; micro-perthite is common. The typical ferro-magnesian mineral is hornblende (horn- blende syenite). Quartz is absent, or, if present, is usually in small bulk in the condition of micrographic intergrowth with the felspar. The hornblende may be replaced to a greater or lesser extent by other minerals, giving rise to what are known respectively as mica -syenite (granite- syenite) and augite-syenite. The most common felspa- thoid- bearing syenite is nephiline- or elaeolite - syenite ; quartz is never found in this variety. When subordinate quartz does occur in syenites, they may be distinguished as quartz-syenite, quartz-mica-syenite, etc. As in • the granites, the syenites may be subdivided into a group rich in potash and another rich in soda.
Chemically syenites resemble granites, but they contain less silica. The specific gravity of these rocks ranges between 2*5 and 3*6. Less variable than granite in its resistance to pressure, the average strength is 1,170 to 1,280 tons per square foot, with a minimum of 730 and a maximum of 2,200.
The absence of quartz renders syenites softer to work than granites, but they are on the whole quite as tough, if not more so. It is only their comparative rarity that prevents their more extended employment for build- ing ; they are excellent in every way, and for polished work they are often superior to granites, on account of the sparsity of mica-flakes. The porosity coefficient is about 1*3. Hirschwald notes that many of the syenite garden-walls in Dresden are upwards of a hundred years old ; the surface of the stone has peeled only slightly here and there, otherwise it is in good condition. It is equal, in fact, in point of wear to the better- grade granites, and is superior to those of medium quality.
84 GEOLOGY OF BUILDING STONES
Although true syenites are limited in occurrence, the name has been used in the stone trade as an appellation for rocks that have no relationship with those described in this section. This is not the fault of the trade alone, for there has been some ambiguity displayed by petrologists in defining the term in earlier years ; indeed, the original ' Syenite,' from the ancient Syene, in the neighbourhood of Assuan, Egypt, is now reckoned a hornblende-biotite- granite, on account of its quartz content. The so-called ' Swedish Syenite,' from Southern Sweden, a coarse- grained green stone, speckled with white, is a diabase ; so also is the ' Lausitz Syenite ' of Spremberg, Prussia ; ' Odenwald Syenite ' is a diorite. All these stones resemble one another in appearance.
DISTRIBUTION. — No true syenite is worked in Great Britain for building stone, though some of the Leicester- shire plutonic rocks approach it in character. The rock occurs in the Channel Islands (St. Helier), in the High- lands of Scotland, and at Llanfaglen and Glan-y-mor in Carnarvonshire. One of the best-known syenites used in Britain is the so-called ' Norwegian Labrador,' the now familiar ' Dark Pearl,' ' Light Pearl,' and ' Imperial Pearl ' of our market ; it is an augite syenite (Laurvigite) from Laurvig and Fredericksvaern. This beautiful stone varies from a rich velvety blue-black to pale bluish-grey; the appearance of sparkling patches is caused by the schiller structure (p. 27) in both felspar and augite crystals. The bulk of this rock consists of felspar (cryptoperthite, ortho- clase, microcline, anorthoclase, and some albite), and with the augite several varieties of amphibole frequently occur along with biotite. Some parts of the rock contain, in addition, olivine and nepheline.
Another Norwegian syenite, Nordmarkite, which occu- pies a large area north of Christiania, consists principally of red micro-perthite, with albite veins, and a little oligo-
OTHER IGNEOUS ROCKS 85
clase ; there is only a small representation of ferro-mag- nesian minerals. This stone is used a good deal in Christiania and the neighbourhood in house construction. Syenites are worked on a small scale in Saxony ; they occur at Monzoni in the Tyrol (monzonite, an augite- syenite), in Southern Portugal, Transylvania, the Urals, and in North America. In the last-named country it is worked to some extent in Arkansas, near Little Rock, where the stone is called Fourche Mountain Granite ; and at Allis Mountain and Magnet Cove an elaeolite syenite (Diamond Jo Granite) has been wrought. The Allis Mountain stone is blue-grey when fresh, but according to Merrill it fades badly ; it has been employed in the cathedral at Little Rock.
Diorite. — This is one of the rocks embraced by the old ambiguous term ' greenstone.' Dark green, indeed, and dark greenish-grey are its prevalent tints ; very compact varieties are nearly black. Diorites are composed of soda-lime felspars, usually oligoclase, andesine, or labra- dorite, with green or brown hornblende. Quartz is absent in the typical rock, but it is frequently present in some varieties, either in fine granophyric intergrowth with felspar or in small grains. Biotite is not uncommon along with hornblende and pale green augite and ensta- tite, or other rhombic pyroxenes occur occasionally. Grains of magnetite, pyrites or ilmenete are usually present, and as secondary minerals resulting from alteration we find calcite, chlorite, and epidote. Structurally the rock is granitic, moderately coarse to fine in grain, and even in texture; porphyritic structure is comparatively rare. In some diorites the hornblende has well-formed crystals, and in others the felspar has crystallized early and has good faces, but more often the crystals are irregular in outline.
86 GEOLOGY OF BUILDING STONES
The diorites are not very well defined rocks, and in nature they pass on the one hand by increase of silica into the grano-diorites and granites, and by decrease of the same material into gabbros. With the introduction of orthoclase they approach the monzonites.
The typical diorite is hornblende- diorite. Other recog- nized varieties are mica-diorite and augite- diorite, those with obvious quartz are quartz-diorites (tonalite).
The specific gravity of diorite is 2*8 to 3 ; the porosity is small, 0*25 per cent. ; the hardness is moderate, and the strength fairly good. Pressure tests give 1,640 to 1,830 tons per square foot. Its weather-resisting quality is fairly high, but the presence of much pyrites tends to lower that of some examples.
It is a difficult rock to work, mainly on account of the absence of a good rift, such as that possessed by granite, and it has not been very extensively employed for architectural work on account of the dull colours of the dressed surface. When polished, however, some of the coarser diorites are extremely handsome rocks, the white felspars showing up strongly in contrast with the dark hornblende, which often forms the greater bulk of the rock. The polish is brilliant and durable, but it is difficult to produce. As a mural decoration and for small columns and monument bases it may be used with very good effect.
A very striking stone is orbicular diorite, Corsite or Napoleonite, from Corsica. In this rock the hornblende and felspar have arranged themselves in large globular masses, i to 2 inches in diameter, with radial structure and concentric rings of alternating dark hornblende and white felspar. The stone is only employed on a small scale for special decorative effects or in small polished articles.
Diorites are limited in occurrence ; in the Channel
OTHER IGNEOUS ROCKS 87
Islands they are found in Jersey at St. Aubyn and St. Clement's Bay, at Fort Tourville in Alderney, and form- ing much of the northern part of Guernsey, where they are extensively quarried, mainly for road material. In England they appear at Hestercombe, near Taunton (tonalite), and at Brazil Wood, near Mount Sorrel in Leicestershire. Diorites occur in South-Eastern Ireland, and they are extensively developed in the Southern Uplands and Highlands of Scotland, but they are nowhere worked for building stones except for purely local pur- poses. There are numerous occurrences in the European Continent and America, but here, too, they are very little wrought.
Gabbro is a rock with granitic structure and a coarse to medium texture. There are many varieties, but the type form is composed essentially of lime-soda felspar, labradorite, or anorthite, with augite. Occasionally the more acid felspars are present in small amount, and in what are called the alkali-gabbros the augite may be replaced to a greater or lesser extent by the rhombic pyroxenes, hypersthene, and bronzite ; olivine, hornblende, and biotite occur in some gabbros as original minerals. Very characteristic of gabbros is the assumption of dial- lage structure by the augite, and schiller structure is of common occurrence, giving rise to play of colour in patches on the broken or polished surface. The py- roxenes, both monoclinic and rhombic, are prone to uralitization or conversion to green or brown horn- blende. Gabbros with much hornblende tend to shade into diorites. Iron minerals and apatite are common accessories.
Classed according to the prevalent mineral, we have gabbro proper, with augite or diallage ; norite or hypers- thene-gabbro, olivine-gabbro, and olivtne-norite ; and when
V \i ' " '
88 GEOLOGY OF BUILDING STONES
quartz is present in microscopic intergrowth, qnartz- gabbro or quartz-norite.
One of the peculiarities of gabbro masses is that they tend to be decidedly patchy in mineral composition, due to local variations ; thus in one part the rock may be wholly built up of felspar (felspar rock, labrador rock, anorthosite) or olivine, diallage or hypersthene, giving rise to oli vine-rock, hypersthene-rock (hyperite), etc.
The specific gravity of gabbros ranges between 2*7 and 3 ; they are fairly hard, and their porosity is low ; their silica content is below 52 per cent. ; and their average resistance to crushing is 1,830, and fluctuates between 640 and 2,200.
They do not weather well, and it is said that those with most felspar (labradorite, etc.) behave the worst, because these crystals decompose readily and produce a rough surface, and thus facilitate the attack of destructive agencies. The colour of the rock is against its employ- ment for architectural building, since it is very sombre when dressed or with rock face. The medium-grained varieties are frequently employed in Europe for road metal. It is put to the most satisfactory use in polished slabs fcr wall-decoration, memorial tablets, small columns, and table-tops. Good large slabs are not easily obtained free from numerous cracks. The presence of schiller, which causes spots and splashes of iridescent purple and blue colour, is a valuable relief to the otherwise gloomy appearance. The effect of a dark dado of this kind is well shown in the stone from Kiev in South Russia, in the Church of the Saviour in Moscow.
Gabbro has been very little used in this country or in Europe for buildings ; the so-called ' Norwegian Gabbro ' is an augite syenite (p. 84). It may be mentioned here that the original gabbro was a serpentinous stone, which received that name locally in the neighbourhood of
OTHER IGNEOUS ROCKS 89
Florence ; what is now understood as gabbro is the grant- torn, or Pietra di maschine, of the same district.
DISTRIBUTION. — Gabbros occur in Great Britain at the Lizard in Cornwall; at Hanter Hill in Herefordshire; St. David's Head in Pembrokeshire ; in the Lleyn Penin- sula, North Wales ; and at Carrock Fell, near Keswick. In Scotland they are found forming laccolites in the Cuellin Hills, Skye, Ardnamurchan, and at many points in the Highlands, and again in Ayrshire. In Ireland they are known about Carlingford ; and in the Channel Isles in Guernsey.
The ' black granite ' of Kentallen Quarry, near Loch Linnhe, is an alkali-gabbro (Kent all enite] ; it contains ortho- clase in addition to plagioclase, along with augite, olivine, and biotite. It is used principally for setts, but it takes a moderate black polish, which exhibits conspicuous mica plates ; when picked or otherwise dressed, it presents a dark dull grey appearance.
In America, as elsewhere, gabbros are not much quarried except for quite local purposes. The ' black granites ' of
[aine from Addison are black, or black with white specks, or dark grey, and of medium texture ; they are obtained in large blocks. The walls surrounding the grounds of the Capitol in Washington are built of this stone ; it also appears in buildings in New York, Boston, Quebec, and Montreal. A finer-grained stone, harder, and of similar colour is quarried at Vinalhaven and Tenant's Harbour. In Minnesota a dark blue-grey gabbro is worked near Duluth (Duluth Granite), together with a highly felspathic variant of
lavender-blue, grey, or green colour, which is wrought into
)lished columns. The lAu Sable Granite' from near Kees- 'ille, Essex Co., New York, and the 'Labrador Granite,'' from Vergennes, Vermont, are norites, coarse in texture, vith bluish schiller ; they are used for polished work.
greenish-black norite is quarried in North Carolina.
GEOLOGY OF BUILDING STONES
Several other basic rocks, often coarsely crystalline, may be mentioned here — e.g., the pyroxenites, mainly pyroxene with subordinate soda-lime felspar ; peridotites and picnics, rocks rich in olivine — which is almost the only constituent in Dunite — associated with diallage, rhombic pyroxene, or hornblende. Eclogite is a hornblende- pyroxene rock con- taining bright green hornblende and augite, with red garnets. These rocks are usually dark green, and are little employed, except in small polished slabs for decora- tive work.
TABLE VII.— PLUTONIC ROCKS.
|
Predominant Alkali Felspar. |
Alkali and Lime-bearing Felspars about equal. |
Lime-bearing Felspar in excess. |
Similar Rocks with Felspathoids. |
||
|
Potash Felspar. |
Soda Felspar. |
||||
|
Acid (not less than 66 per cent, silica) |
Granites rich in potash |
Granites rich in soda |
Adamellite granite |
Granodiorite |
— |
|
Intermediate mot less than 52 per cent, silica) |
Syenite rich in potash |
Syenites rich in soda |
Monzonite |
Diorite |
Syenite with Nepheline |
|
Basic (less than 52 per cent.silicaj |
(Shonkinite, etc.) |
(Essexite, etc.) |
Kentallenite |
Gabbro |
Gabbro with Nepheline |
HYPABYSSAL ROCKS.
Quartz Porphyry, some felsites, granophyres, and pitch- stones correspond to the granites.
Porphyry, orthoclase-porphyry, syenite-porphyry, nephe- line-syenite porphyry, etc., correspond to the syenites.
Porphyrites correspond to the diorites.
Dolerites and most Diabases correspond to the gabbros.
OTHER IGNEOUS ROCKS 91
No little difficulty is experienced in framing a brief comprehensive explanation of the rocks in this class for readers unversed in the methods and outlook of petro- logists. Perhaps their position may be indicated most briefly by saying that they are intrusive igneous rocks which have cooled more rapidly than their plutonic equivalents.
Now, just as there have been all degrees in the rate of cooling in magmas of every grade of chemical composi- tion, from acid, through intermediate to basic, so we find in Nature gradual passages from each kind of hypabyssal rock into its plutonic and volcanic analogue, and the same kind of gradation between the most acid and the most basic forms. Since architects and engineers are less con- cerned with the mode of origin than with the structure, texture, and mineral contents, and the correlated physical and chemical properties, the question will naturally be asked, What are the characteristic structures and other properties by which stones quarried from hypabyssal rock masses can be recognized? It may as well be said at once that there is no satisfactory direct answer to this question.
Structurally they range from holocrystalline granular to glassy non-crystalline rocks ; in texture they vary from medium coarseness to very fine or vitreous ; but, on the whole, their mineral ingredients are crystallized on a smaller scale than in the plutonic representatives ; they frequently, but not essentially, contain one or more porphy- ritic minerals, and spherulitic structures are frequently developed. Although they bulk less largely in the earth's crust than the plutonic or volcanic rocks, they appear in many more recognizable variations- A few types will now be described,
Porphyry. — Under this head we have to consider two distinct sub-classes, the Quartz-porphyries related chemi- cally to granites, and Porphyries proper, similarly associated
92 GEOLOGY OF BUILDING STONES
with the syenites. The characteristic felspar of both porphyry and quartz-porphyry is orthoclase.
Quartz-porphyry is a rock consisting of a fine-grained micro -crystalline or cryptocrystalline (felsitic) ground mass composed of quartz and felspar, forming a compact- locking base. In micro-crystalline structure the individual grains can be distinguished by the microscope, but in the cryptocrystalline structure the individuals are indistin- guishable ; all that can be recognized is that the material is crystalline. In this ground mass there are embedded large crystals of felspar and quartz in well- formed crystals, or in rounded lumps the size of a pin-head to that of a bean. With these, but subordinate in number, there may also be crystals of mica, most often biotite, sometimes pale green augite, green hornblende, or blue riebeckite. If the larger porphyritic crystals are absent, the rock is sometimes called a felsit; the felspathic constituent being orthoclase, albite, or anorthoclase. Varieties of felsite rich in soda are Kcratophyre and Quartz-keratophyre.
Quartz-porphyries in which the micro-graphic and cryptographic structures are particularly well developed are called Gran* phy res; they may or may not have porphy- ritic felspar and quartz.
When the structure of the quartz-porphyries is micro- granitic — like granite on a small scale — the rock is a granite-porphyry or micr <. -granite ; many of the clvans of Cornwall are of this character.
The glassy variant of quartz-porphyry is Pitchstone, a rock consisting almost entirely of glassy matter, in which tiny incipient crystals (crystallites, microlites, etc.) are revealed by the aid of a microscope. In this glass there may be porphyritic crystals of orthoclase, quartz, mica augite, and hornblende. Pitchstones have a resinous lustre, and conchoidal fracture and flow-structure is com- monly visible, either with the naked eye or with a lens.
OTHER IGNEOUS ROCKS 93
Minute perlitic (subconcentric) cracks are nearly always present. The colour of the rock is dark green, grey- black, occasionally red.
Porphyry is similar in structure to quartz-porphyry, but it belongs to the intermediate group of rocks on chemical grounds ; as in the syenites, quartz is typically absent or feebly represented. Varieties are syenite-porphyry, nephe- line-porphyry, nepheline-syenite-porphyry, etc.
The several varieties of quartz-porphyry and the por- phyries are usually of a reddish colour, sometimes pale grey, yellowish, or light brown. They are mostly com- pact stones, rarely porous, or containing cavities. Their specific gravity is 2*4 to 2*8. The pressure tests of fresh material show high values for the resistance to this kind of stress; the average. is 1,645 tons per square foot, and they go as high as 2,740. The average porosity is 0*65 per cent. (0*3 to I'o). Absorption under normal pressure is between 0*9 and 3*5 per cent, of the weight. Their weather-resisting properties are generally very good. Their hardness and weather-resisting power depend largely on the amount of silicification of the ground mass ; much of this silicification may be of secondary origin. The stones may be quite good, even if the felspathic part shows indication of alteration, but in the quartz-free porphyry any indication of earthy appearance on the freshly-fractured surface should cause some distrust of the stone.
Although they are hard rocks and tough, they are not really so difficult to quarry as is sometimes supposed, but the absence of good rift frequently makes them more troublesome to obtain than granites. Most of them take a polish readily.
DISTRIBUTION. — In Cornwall and Devon there are many micro -granites and quartz -porphyries associated with the granite masses ; they penetrate both the granite and the surrounding rocks. Locally they are called
94 GEOLOGY OF BUILDING STONES
elvans, but the name is applied also to the more basic dykes and to some of the finer-grained granites.
The Pentuan (Pentewan) elvan was formerly a favourite stone for ecclesiastical work in the West of England, both for exteriors and interiors, and was employed in other buildings near the quarry in Pentuan Bay, Cornwall. The Roman inscribed stone built into the wall in Tregony Church, is a rough block of this material. The stone has a pale buff tint, and is fine-grained and rather porous. It probably owes its good qualities as a weather-resisting stone to the conversion of the ground mass to an aggre- gate of quartz and white mica (greisenization). The por- phyritic felspars are a good deal kaolinized and silicified.
An altered quartz-porphyry of a warm yellow colour is quarried near St. Columb Minor, and used in the parish church and houses in the village, and in Newquay for lintels, quoins, and facings. The well - selected stone wears very well, turning to a warm grey.
A coarsely porphyritic micro - granite is quarried at Retyn in large blocks; it would look well polished. At Tremore Quarries, near Bodrnin, a broad dyke is largely quarried ; it varies much in structure and texture in different parts : that from the centre is more coarsely crystalline, and is usually employed for building and polished work. The appearance varies from dark pinkish- red speckled with black to a pale pinkish-grey speckled black and white. Some varieties of this stone are now wrongly sold as ' Luxullianite.' The pale greenish-grey elvan from Newham has been extensively used in Truro.
Many other acid elvans have been quarried from time to time for local buildings, but mainly for road metal — e.g., at Mayon ; Land's End ; Dowglas, near St. Austell ; Helland ; St. Neots; Lanivet and Withiel, near Bodmin ; Trevailes and Rosecraw, near Penrhyn, etc. They are usually too much jointed to be obtainable in large blocks ;
OTHER IGNEOUS ROCKS 95
many of them show much secondary alteration, kaoliniza- tion, and silicification, and in the latter case they weather well. A good collection of these stones is in the Jermyn Street Museum.
In South Wales micro-granites and granophyres occur at St. David's. Micro-granite, granophyre, and granite- porphyry are plentiful in North Wales in Carnarvonshire, Merionethshire, and Anglesey. A large mass of grano- phyre lies between Wastwater, Ennerdale, and Butter- mere, in the Lake District ; the dark mineral in this rock is mainly biotite. A granite-porphyry, with porphyritic orthoclase and small garnets, is quarried at Threlkeld. In the same district there are many other minor occurrences of similar rocks, mostly in the form of dykes and sills.
In Scotland bosses and dykes of quartz - porphyry are very abundant, associated in many cases with the granite masses. The rock quarried at Ailsa Craig contains riebeckite.
Quartz-porphyries, micro-granites, and felsites occur in Ireland, round the granite of Leinster, and again in North Galway ; pitchstones and felsites in the Barnsmore range, Donegal.
On the Continent these rocks occur at numerous places — e.g., in Silesia, Saxony, the Odenwald, Thuringia, the west border of the Black Forest, Vosges, and the Rhine Provinces. In the Bozen district in the Tyrol a greenish- brown and reddish rock is quarried. From Sweden may be mentioned the red felsite of Elfdalen, which polishes well ; and from Norway the brown rock, with red porphyritic orthoclase, from the Christiania district. In Egypt there are many quarries in the Nile Valley.
In America there are numerous occurrences of these rocks in Massachusetts (bright red, pink, and grey), Missouri, Minnesota, Maine, New Hampshire, Nevada, and Wisconsin ; but they are regarded as being very
o6 GEOLOGY OF BUILDING STONES
difficult to work, and are little used except for local rough structures.
The fine-grained granitic texture of the micro-granites, the close interlocking of the minerals in the granophyres, and the feltwork of the small crystals in the quartz- porphyries and felsites, all tend in the direction of im- parting toughness to these stones; hence they form capital material for foundations and heavy work in embankments, bridges, etc. When polished they have been used for internal wall covering, mantelpieces, columns, landings, and bases for monuments.
Greenish -grey and red granite-porphyries have been used in the basin of the Springbrunnen in the Schlacht- haus Hof and the Universitatsfrauenklinik in Leipzig; in the Reichspatentsamt and Reichsversicherungsamt buildings in Berlin.
Porphyrite. — The essential difference between these rocks and the porphyries is that in the former the charac- teristic porphyritic felspar is a lime-bearing plagioclase.
In all other respects, as regards their prevalent structure and physical properties, they closely resemble the por- phyries, and behave similarly in buildings. Less bright in colour than most of the porphyries, they may, never- theless, be employed in a like manner in polished work with good effect.
DISTRIBUTION. — Porphyrites occur in Somerset at Moons Hill Quarry and Beacon Hill, near Shepton Mallet. In Leicestershire the well-known roadstones of Markfield, Groby, Cliff Hill, etc., belong to this class. They have a granophyric ground mass, with porphyritic plagioclase and green hornblende, part of which is an original constituent, and part is an alteration product of original augite. These stones are extremely tough, and are much used for road material. They approach the
OTHER IGNEOUS ROCKS 97
diorites in some of their characters, and may be compared with the similar stones from the Cleveland dyke and Carrock Fell rocks, and with the Penmaenmawr stone from the coast of Carnarvonshire. Similar rocks, with hyper- sthene, come from Cam Boduan, near Nevin, and, with mica in addition to hypersthene, from Yr Eifl, in the Lleyn Peninsula; this is a well-known roadstone. In South Wales rocks of this kind, with augite and hornblende, occur near St. David's.
In Scotland there are many occurrences of porphyritic dykes ; they are little used for building, but are worked here and there for road metal.
In Ireland the best-known porphyrite comes from Lambay Island, off the Dublin coast ; it is usually styled ' Lambay porphyry,' and is an ideal example of a por- phyritic rock. The ground mass is dark green, composed of small lath - shaped felspars, with grains of augite ; it is an augite -porphyrite. The rock has undergone a good deal of alteration, with the consequent formation of chlorite, calcite, epidote, and magnetite. Scattered throughout the ground mass are porphyritic crystals of pale green labradorite felspar. This is a handsome stone, and takes a fair polish ; it closely resembles the rock known as porfido verde antico. It is difficult to obtain.
Porphyrites occur at several places in Saxony, Hartz Mountains, Thuringia, Riesengebirge, Black Forest, and Vosges.
The Belgian rock from the Quenast quarries and Lessines is a familiar road metal in England ; it is a quartz-porphyrite or quartz-diorite-porphyrite. It is very tough, and the Quenast rock has a crushing strength of 2,520 tons per square foot.
Dolerites and Diabases. — For present purposes these two rocks may well be regarded as one type, since the
7
98 GEOLOGY OF BUILDING STONES
main difference between them is that diabases usually show more alteration of the minerals than is the case in the dolerites. Moreover, the term * diabase ' has been employed very loosely in this country, while in Germany and in America it is used with a somewhat different connotation.
Dolerites are composed essentially of plagioclase felspar, most commonly labradorite, but sometimes oligoclase and anorthite, together with augite. To these may be added the occasional appearance of other minerals, which give rise to the varieties olivine-dolerite, mica-dolerite, hyper- sthene-dolerite, and, if a little quartz is present, quartz- dolerite. Chlorite, epidote, calcite, and sometimes ser- pentine, appear as alteration products ; pyrites, and more rarely magnetite, are usually present.
The structure is fairly constant in character and some- what peculiar (Fig. 3). The felspars in lath-shaped crystals have usually crystallized before the augite, which forms large irregular crystals, enclosing a number of the felspars ; this structure is known as ophitic. Instead of having this ophitic structure, the augite may appear in the form of roundish grains between the felspars. The rock is nearly always completely crystalline, but occasionally small residual patches of glass remain. Porphyritic struc- ture is rare. The texture is medium-grained to fine- grained. Sometimes amygdaloidal cavities occur.
When freshly quarried the rock is very dark, greenish or bluish black ; but it soon becomes greener on exposure, owing in some cases to the drying of the serpentine, which occurs as an alteration product of olivine. The dolerites and diabases have often been called green- stones. When weathered the surface tends to assume a dull brown tint, on account of the liberation of hydrox- ides of iron. The felspars are often stained a greenish tint, but sometimes they retain their pale colour, in which
OTHER IGNEOUS ROCKS 99
case the trade name * syenite ' has been applied to the stone.
The specific gravity of these stones is about 2*8 to 3. The crushing strength is about that of the mean for granites, and is somewhat lower than that of diorites.
The weathering qualities are good on the whole, but less so in the coarser varieties. The stone is rather more difficult to work than granite. It is frequently well jointed, but there is seldom any definite rift. A high polish is difficult to attain, but where a subdued colour and smooth surface is required it may be very usefully employed.
Dolerites and diabases are more often used for road metal than for building, principally because their dull colour is not appreciated by architects, although when relieved by juxtaposition with brick or lighter coloured stone, they may have a good effect. In the polished state they have been used for walls and monuments.
DISTRIBUTION. — Many of the greenstone dykes$ sills, and small bosses of Cornwall and Devon are dolerites and diabases ; they are quarried in many places for roadstone, but in building are mainly employed in rough walling. We may mention here the large diabase quarries at Stepper, near Padstow, which produce a compact dark green rock of fairly coarse grain, and the Catacluse stone from near Trevose Head, Cornwall, a dark, compact bluish- grey stone. Good examples of the use of this stone may be seen in many Cornish churches, e.g., the old font in St. Merryn's Church.
Polyphant stone, although not a dolerite, may be in- cluded here ; it is a peculiarly altered peridotite, ob- tained near Launceston. It has been extensively used for churches in the past for dressings and for carved work, and for fonts inside. It is a dark green stone, with pale green spots of felspar, which take on a warm brown
ioo GEOLOGY OF BUILDING STONES
tinge in the weathered samples. The stone is tough and wears well in dry situations, but it is extremely soft and readily carved. At the present day it is being extensively used for ornamental carved and polished work. The copy of an old Cornish cross, forming the War Memorial at Haverfordwest, may be taken as an example.
' Trusham granite ' is a dolerite from near Trusham in Devon, and is worked for roadstone and setts ; but it might be polished in blocks of moderate size, as the colour in this condition is a quiet dark green.
In the Midland counties they are well developed about Nuneaton ; at Atherstone (hornblende-dolerite) ; Hartshill ; the Lickey Hills; Bayston Hill, near Shrewsbury; Lower Wych Quarry, in the Malvern Hills ; Ponk Hill, near Walsall ; Rowley Regis, near Birmingham (Rowley Rag) ; the Clee Hills; and other places in the neighbourhood; and in the Welsh border counties. In Derbyshire dolerites form sills in the Carboniferous limestone at Ible, Peak Forest, Tideswell Dale, and elsewhere (locally = ' toadstone ').
In South Wales dolerites are abundant in Pembrokeshire about St. David's, Fishguard, and Newport ; and in North Wales they are prevalent in the counties of Merioneth and Carnarvon. The Gimlet rock at Pwllheli is one of these, quarried for macadam and setts ; in its coarser varieties it is also a very handsome stone when polished, having pale felspars sprinkled through a dark green ground. Carth Head, near Keswick, is a diabase rock, and other examples occur in the Lake District.
Very well known dolerite dykes of great length occur in the North of England — for example, the Cleveland dyke, in Yorkshire, traceable for ninety miles ; the Great Whin Sill, stretching from Dunstanburgh Head in Northumber- land to Cross Fell, seventy miles ; the Acklington dyke, from the Northumberland coast to the Cheviots. There are also manv smaller ones. These stones have been
OTHER IGNEOUS ROCKS 101
used considerably in local building, and bushes for water- wheels have been made of this material.
In Scotland dolerites and diabases are widely scattered ; they are worked mainly for road metal at many localities.
In Ireland they appear in Wicklow, Waterford, and Antrim ; and there are several small bosses in the Isle of Man.
In America these rocks are quarried in Connecticut, Massachusetts, New Jersey, Virginia, and Pennsylvania, principally for road material. In the last-named State there are important quarries at Collins Station, where the stone is worked for monuments and foundations, and, in a lesser degree, for other building purposes ; also at York- haven, York County, whence much stone has been taken for the North Central Railway for bridges, etc. The ' Palisades,' or dolerite cliffs of the Hudson River, New Jersey, have been much quarried for road metal and setts (' specification ' blocks, 4 inches by 8 inches by 10 inches, and 7 to 8 inches deep, and square blocks 5 to 6 inches square, and 6 to 7 inches deep). St. Patrick's Cathedral, Hudson County House, and other buildings in New Jersey, have employed this stone.
Dolerites are obtained near Eisenach, Lauterbach in Meiszen, in Oberhesse, Eifel, Kaiserstuhl, Lowenberg in the Siebengebirge, Oberhausen in Amstal, Lausitz, and a fine-grained variety at Brockenham, near Frankfort-on- Main. Saxon diabases have been frequently employed in monumental bases and family vaults. The obelisk Kriegerdenkmal in Bautzen, Saxony, and Kaiserdenkmal in Pasewalk, Pomerania, are of diabase.
VOLCANIC ROCKS.
The volcanic rocks now to be briefly described include lava flows of all ages, from -the most ancient to quite recent. Their composition, as the table on p. 102 indicates,
102
GEOLOGY OF BUILDING STONES
shows a wide range of variation; and in texture they show similar diversity, from compact dense basalts to glass-like obsidian and frothy pumice. When fresh they range in colour from white, through neutral greys, to black ; but with subsequent alterations they change, in the more basic kinds, to dull browns and greens.
TABLE VIII.— VOLCANIC ROCKS.
|
Potash- |
Soda- |
Lime- |
With |
||
|
bearing. |
bearing. |
bearing. |
Felspathoids. |
||
|
Acid (not less |
Potash |
Soda |
i"^ |
Dacite |
_ |
|
than 66 per |
Rhyolite |
Rhyolite |
ro p % ^ |
||
|
cent. SiO2) |
• v <u |
||||
|
I! |
|||||
|
Intermediate |
Potash |
Soda |
?l |
Andesite |
Phonolite |
|
(not less than |
Trachyte |
Trachyte |
3 |
||
|
52 per cent. |
|||||
|
Si02) |
• •^ 2 |
||||
|
--< >3 en ^8-1 |
|||||
|
Basic (less than 52 per cent. Si02) |
— |
— |
13| .2^2 £ .S cL |
Basalt |
Leucite, Nepheline, and Mellilite |
|
c^ |
Basalts |
Rhyolites are mostly pale grey rocks with a ground mass that may consist entirely of microlites of felspar with quartz, or there may be, in addition, more or less unresolved glassy matter ; if the ground mass is mainly glass the rock is an Obsidian. Many of the rocks called felsites belong to this class, for their characteristic felt- work of felspar needles and microlites has resulted from the devitrification of glassy rock, much in the same way that old Roman glass has altered, or modern bottles will devitrify if placed in suitable environment. Flow structure and banded structure, spherulitic structure and small subconchoidal (perlitic) cracks are very characteristic. Porphyritic structure is common ; the porphyritic crystals
OTHER IGNEOUS ROCKS 103
are quartz and sanidine (sometimes albite, anorthpclase, and oligoclase), with plates of dark mica and prisms of augite or hornblende in smaller numbers (Fig. 3, V).
Trachytes resemble rhyolites very closely in external features ; the chief difference lies in their chemical com- position, which places them in the intermediate class. Like the rhyolites they are grey in colour, except when tinted green or pale brown in older occurrences ; they often show marked flow structure ; on the other hand, they tend to be less vesicular, although they often have drusy cavities. The porphyritic crystals are sanidine, anorthoclase, oligoclase or andesine, and brown mica, green or blue amphibole, or one of the pale pyroxenes. Quartz is much scarcer in the trachytes than in the rhyolites. The ground mass is of the felsitic type, and is usually more completely crystalline than in the rhyolites. According to the predominant ferro-magnesian mineral, we have amphibole-trachytcs, pyroxene - trachytes, and mica-trachytes.
Phonolites are comparatively rare rocks ; they resemble trachytes, but differ in containing a higher alkali content, and in the presence of felspathoids in addition to sanidine. They are grey or greenish-grey in colour, and compact. When thin plates of the rock are struck sharply they give out a metallic or bell-like sound, which has given rise to the name ' clinkstone.'
Andesites, unlike the preceding rocks, are characterized by porphyritic plagioclase of the soda - lime series, andesine, labradorite, sometimes oligoclase ; with the felspar there may be biotite, light green augite, hyper- sthene or hornblende, the last-named being the most common.. Hence there are hornblende-and^sites, mica.-
io4 GEOLOGY OF BUILDING STONES
andesites, augite- or hypersthene-andesites ; and if a little quartz is present, quartz-andesites. These pass into dacites, if more than a small amount of quartz occurs. Porphyritic structure is usually well marked in andesites ; these crystals lie in a ground mass composed most often of a felted aggregation of felspar needles and microlites, in which a certain amount of glassy matter is enclosed. The glassy matter may be wanting, or it may assume large proportions, as in ' andesite glass.' Flow structures are occasionally evident, and vesicular or amygdaloidal structure is common. When fresh the rock is dark in colour, and the porphyritic felspars are clear and glass- like, resembling sanidine ; but both ground mass and porphyritic felspars are commonly stained by alteration products ; chlorite produces greens, and hydroxides of iron give rise to browns.
From a technical point of view these rocks — rhyolites, trachytes, and andesites — may for brevity be considered together. They are widely employed as general building materials on the Continent, but, owing to the great range of porosity and the variability in texture, it is not easy on the small amount of evidence to indicate any guiding principles for their employment.
Rhyolites rich in quartz and compact in texture seem to weather better than the more porous or glassy varieties. None of them take a good polish, though obsidian is treated in this way for small ornamental objects. In the younger rhyolites the sanidine crystals, having very often smooth, clean faces, tend to fall out from the weathered surfaces of dressed stone ; in the older ones, however, this is not the case, because they become intergrown with the ground mass; and in many cases the more ancient rhyolites are stronger and more weather - resisting on
OTHER IGNEOUS ROCKS 105
account of the formation of secondary quartz within the rock. The same feature may be noticed in older trachytes and andesites.
Vesicular structure makes for lightness ; it reaches its highest expression in the rhyolites, in the rock pumice. The specific gravity of this stone in the mass is 0*3 to 0*9 ; for this reason it is not bad material for vaults and arches, and it has been employed for this purpose by the Romans. The cupola of Agia Sophia, Constantinople, is made of pumice, and in the Lipari Islands, where it is readily obtained, it is employed in many buildings in roughly- carved blocks set in mortar. All the porous or rough varieties of rhyolite, trachyte, and andesites form a good bond with mortar. Mixed with milk of lime crushed pumice is used for the artificial stone ' Niedermendig Schwammstein,' which is employed as a heat-insulator. Crushed pumice alone is used in New Zealand and Southern Europe for steam-packing, heat-insulating, also as a floor-packing.
The behaviour of rhyolites, etc., in buildings is vari- able : some harden considerably on exposure, others do not.
The castle of Pfalz, Kaiserwerth, 1,100 years old, is built of a compact, well-crystallized quartzose rhyolite, and, probably on account of the quartz, it is still in good preservation. On the other hand, a rather porous rhyolite, with ill-defined sanidine crystals in a fine matrix, employed in the renovation of part of Cologne Cathedral, has peeled badly after only fifteen years' exposure.
As a useful stone phonolite is of only local significance ; it weathers pretty satisfactorily, and its crushing strength is about equal to that of the andesites and trachytes ; it sometimes reaches 1,830 tons per square foot; its specific gravity is 2-5 to 27. Both in weathering and quarrying it tends to break into thin slabs, a feature which makes
106 GEOLOGY OF BUILDING STONES
it handy for walling and paving stones ; in the Auvergne it has even been employed for roofing.
Trachytes and andesites give only moderate results under crushing tests— 455 to 730 tons per square foot ; their hardness is variable, and usually only moderate ; the specific gravity is about 2*2 to 2*7, and the porosity of compact varieties about 3 per cent, to 6 per cent, in more porous kinds. The compacter, less porphyritic varieties seem to weather best, for the large felspars give way rather easily. Andesites that are very amygdaloidal are of doubtful character.
Both trachytes and andesites are used in building a good deal in Europe, but not in England or America. They are not difficult to work in moderate-sized blocks, and they key well with mortar. They are used for ashlar, steps, and landings, small columns, balusters, and archi- traves. As macadam and for local building they are quarried in Scotland, and they are rather favoured on the Continent for cobbles, or kleinpflaster, in the steeper streets with light traffic, on account of the good foothold they afford.
Rhyolites, trachytes, and andesites are quarried in England and Scotland, Wales and Ireland, at many places for roadstone.
Trachytes and andesites occur in the Lipari Islands, in Central Italy, the Siebengebirge, Steirmark, and Iceland. Occasionally they are used as millstones. They are quarried in the Siebengebirge district in Germany ; in the Rhone Valley (Altberg) ; in the Eifel district (Kalberg) ; in the Westerwald, Vosges, and Auvergne. They are quarried in a small way locally in America. In Servia phonolites are worked at Banjvea, andesites at Dobra, trachytes at Kopaonik.
Basalts. — These well-known rocks are typically com- posed of basic plagioclase felspar (mainly labradorite in
OTHER IGNEOUS ROCKS 107
the bulk of the rock, with anorthite or bytownite occurring as porphyritic crystals) and augite. To these must be added olivine, which is a very common porphyritic con- stituent, and occasionally hornblende and mica ; grains of quartz are rare. The iron minerals, magnetite, ilmenite, and iron pyrites, are invariably present, and sometimes haematite, and the rock is often much altered, with the accompaniment of calcite, serpentine, and chlorite.
Basalts occur as lava flows of all ages, and in structure they may be occasionally quite glassy (tachylyte), or have a glassy base full of small lath-shaped felspars and small grains of augite, or there may be no glassy matter left. In this holocrystalline condition they tend to pass into dolerites — e.g., the dolerite of Linz-on-Rhine. Porphy- ritic structure may be present or absent, and the same applies to vesicular and amygdaloidal structure. Basalts with the last-named characters are not suitable for em- ployment.
The colour of basalts when fresh is black to dark greenish-grey ; when weathered or altered, they take on warm brown tints or dull dark greens. The fracture of fresh varieties is uneven or conchoidal ; they are tolerably hard and very tough. Their specific gravity is 2*8 to 3*3, and the crushing strength is very high — 2,010 to 3,200 tons per square foot, and it may reach 4,500 tons per square foot. The absorption is about i per cent., and the conductivity to heat is considerable, with the natural result that basalt buildings are cold in winter and hot in summer, and this peculiarity is intensified by their dark colour.
Basalts are not particularly pleasing when used archi- tecturally on account of their sombre hues, and except in a few places they have not been much used in this type of work.
The felspar basalts, free from glass and rich in granular augite, give the best results; in buildings 700 years old
io8 GEOLOGY OF BUILDING STONES
the decomposed surface in such cases is only I to 2 milli- metres thick, with bright red spots where the augite has broken down. Even porous varieties show very satisfac- tory weather results ; thus a 6oo-year-old wall of Nieder- mendig ashlar exhibited only an insignificant weathered crust, and the stone was quite sound internally. Again, an altered basalt of light grey colour, due to abundant secondary alteration products, from the Roman castle of Saalburg, near Homburg, 1,700 years old, is proved to be still quite strong.
In damp situations and under water basalt does not behave so well ; decomposition penetrates farther into the stone, and forms a brown clayey crust. Hirschwald states that out of a number of basalt structures in air 85 per cent, were found very good, and the remaining 15 per cent, in fairly good condition ; whereas of basalt structures subjected to the continued influence of water, 50 per cent, had turned out badly, 15 per cent, were moderately sound, and 35 per cent, were well preserved.
Both compact and other varieties on copings and cor- nices show a tendency to crack.
The felspathoid basalts wear well enough if the minerals are fresh to begin with ; should they show signs of altera- tion in the quarry, they are not likely to stand in the buildings.
DISTRIBUTION. — In Great Britain basalts are rarely used for anything better than rough boundary walls and roadstone. They are found in two or three localities in Cornwall ; in Devon, about Ashprington, Totnes, Brent Tor (near Tavistock), Ide (near Exeter) ; and at numerous places in the palaeozoic rocks about St. David's in Pem- brokeshire. They appear at a few places in Somerset- shire and Shropshire ; at Eycott Hill, near Keswick, and at other points in the North of England. In Scotland prominent examples are Arthur's Seat, the Lion's Haunch
OTHER IGNEOUS ROCKS 109
and Calton Hill, Edinburgh ; also there are many other occurrences in Haddingtonshire, Fifeshire, Linlithgow- shire, Ayrshire, and in the islands Skye, Rum, Mull, Arran, Raasey, and Eigg. In the Isle of Man they are found at the Stack of Scarlet, and in Ireland they form the familiar Giant's Causeway of Antrim, and occur in Limerick and a few other localities.
Abroad, basalts are widely dispersed, but rarely used extensively for building. They are found in Bohemia, Silesia, Lausitz, the Erzgebirge and Fichtelgebirge, Thuringewald, the Rhone Valley, Hesse (Kamel, Meizner, Habichtswald), Vogelsberg, Odenwald, Black Forest, Siebengebirge ; also in Central France, North Italy, Switzerland, and Hungary. In America there are num- erous occurrences, but the stone is hardly ever employed for building. Basalts have been employed on the Conti- nent for external walls, foundations, steps, columns, and socles in architecture, also for retaining walls, bridge-piers, dams, etc. They have been much used for macadam.
Very common in basalts is columnar structure ; the columns range from the thickness of a lead-pencil up to 3 or 4 feet in diameter, and as much as 40 feet in length. This character is well exhibited in Fingal's Cave, Staffa, at the Giant's Causeway, and in quarries at Lausitz, Meiszen, Habichtswald, and many in the Rhine district. This structure facilitates quarrying and the columnar blocks are frequently employed transversely in walls and in em- bankments (Rhine), without dressing, also for paving, and as direction and boundary posts.
Volcanic Tuffs. — In recent times the expulsion of fine dust-like material, together with small lumps and large blocks of rock, from active volcanoes is a familiar pheno- menon. We may cite, for example, the great outburst of
no GEOLOGY OF BUILDING STONES
Krakatau in 1883 ; and, going farther back, we may point to the material which effaced for so long the town of Pompeii. These materials, whether they fell on land or water, settled down after the manner of sedimentary deposits. The younger tuffs generally form light porous stones, but those of earlier geological periods are fre- quently— indeed, usually — in a much more compact and solid condition through silicification and other mineral alterations that have taken place in them. Among these fragrnental volcanic rocks there are coarse varieties — vol- canic agglomerates and breccias — rocks of medium texture, and others made of the finest of dust.
They may be classified simply according to the nature of the magma from which they had their origin ; thus, there are rhyolite-tuffs, trachyte-, andesite-, phonolite-, and basalt - tuffs ; a peculiar basic variety is called pala- gonite-tuff.
In the older rocks, when the original fragmentary material has undergone change, due to mineralization and earth pressures, very compact rocks result ; some of the stones known as ' porcellanite ' and * halleflinta ' are of this nature ; others that have become markedly sheared become slates (p. 274). The fragments forming tuffs are very varied ; they are very often glassy, and, in addition to the volcanic matter, they may contain chips and dust of any kind of sedimentary rock ; even fossils may occur in them. They are often highly calcareous. The tuff of Brohltal is used for making hydraulic lime.
As building stones the tuffs naturally vary much in quality : those with the simplest composition, compact and uniformly glassy, are probably the best ; phonolitic and other felspathoid tuffs are poor. It is a common occur- rence to find a distinct hardening on the exposed surface of dressed stones, but this is not necessarily an indication of good quality.
OTHER IGNEOUS ROCKS in
Unaltered tuffs are very porous ; in some the pores are nearly half the volume of the stone. It is an interesting commentary on the loose generalization that the porosity of a building stone is an index of its frost-resisting power to find that many of the porous tuffs are less affected by frost than more compact kinds. Most unaltered tuffs are very easy to work and of low specific gravity, and they are less affected by lichens than limestones.
The crushing strength is variable, usually low: that of the Wieburn tuff is 130 tons per square foot, the flesh- coloured porphyry-tuff of Rochlitz, in Saxony, a very good weather stone, has a mean value of 180 tons per square foot ; a dirty green fine-grained diabase tuff from Hof, in Upper Franconia, has a specific gravity of 2 '8 to 3, and a crushing strength of 1,370 tons per square foot : this is also a good stone.
The trachyte tuff of the Eifel and Siebengebirge is only fit for building in parts — mostly it is too friable ; it is often called Trass (Andernach and Nettetal), and is largely em- ployed in the manufacture of cement. Tuffs are repre- sented in Great Britain in most of the districts where the older volcanic rocks occur ; they are almost always much altered, hardened and sheared, and occasionally used for roadstone (and slates).
CHAPTER V
SANDSTONES AND GRITS
THE degradation and decomposition of rocks like the granites and gneisses gives rise to the formation of quan- tities of fragmentary material, which in course of time, by the continuation of the disruptive and solvent weathering agencies, and by the transporting action of water in its several capacities, assisted in places by the wind, becomes sifted and sorted and laid down in deposits of different grades of fineness and of diverse composition, in beds or strata.
If for the sake of simplicity, we confine our attention to the behaviour of a typical granite and its degradation products in a moist temperate climate, we shall be able to observe all the essential stages in the production of strati- fied sedimentary rocks — namely, the sandstones and their allies, the clays and their derivatives, the limestones, and the multitude of deposits of intermediate composition that link these three outstanding classes one with another.
In a granite quarry where there is a considerable amount of over-burden, waste, or ' head ' visible, the solid, close- grained rock, some depth from the surface, may be observed to pass upwards into a more openly-jointed stage, when the irregular joints are seen to be more and more occupied by rotten granite material as the surface is approached. The effect of the rotting of the granite along the joints, together with the multiplication of the joints near the surface, is to break up the higher layers of the granite mass into a col-
112
SANDSTONES AND GRITS 113
lection of isolated cores or lumps of undecomposed rock, surrounded by incoherent material. As the subsoil is approached, the size of the loose blocks tends to decrease, and finally, in the higher subsoil, the bulk of the material is incoherent granitic rubbish, capped by soil.
When these places are examined in more detail, it will be found that the low-lying friable granite differs little in appearance from the compact parent rock. The quartz, felspar, and mica are easily distinguishable ; preliminary chemical changes have led to mechanical disruption, but as the material is traced towards the overlying soil, it becomes increasingly pasty and clayey in character, fewer felspar fragments are recognizable, and the colour is usually some shade of brown or yellow. The changes produced in the lower subsoil through the agency of surface water and oxygen are continued with increased rapidity in the region of the soil where the larger plants and microscopic bacteria, aided by the earthworms and other animals, continually operate destructively upon the rock particles.
When such comminuted and decomposed products of he original granite come, as sooner or later they must, under the influence of moving water, they are subjected to prolonged and oft-repeated processes of transportation, with continual degradation and sorting of the particles, until conditions are favourable for their final deposition, in beds of uniform character, on the floor of some sea, lake, or estuary, or the flood-banks of a stream.
Should conditions favourable to deposition arise near the site of the parent rock, the transported material may have suffered so little sorting and modification that the deposited fragments may represent the quartz, felspar, mica, and subordinate ingredients of the granite almost in their original proportion. Such a deposit after con- solidation is known as an arkose ; it will differ scarcely
8
ii4 GEOLOGY OF BUILDING STONES
at all from the original granite in chemical composition, though the mode of aggregation of the particles is dif- ferent. Many of the grits of Carboniferous and Old Red Sandstone age and older palaeozoic rocks are of this nature. At an early stage in the process of transportation the finer, clayey, pasty particles — largely the product of the break- ing down of the felspars — are washed free from the coarser grains of quartz and carried farther ; so that it is usual to find mud deposits in one place, sandy quartz debris, with some undecomposed felspar, in another.
The coarser quartz grains, when aggregated and consoli- dated, form beds of Grit ; the finer-grained quartz waste goes to form beds of Sandstone. If by any chance a permanent deposit is formed near the original granite source containing angular blocks of still undisintegrated granite, we should have a Granite Breccia ; should the deposit contain rounded blocks, pebbles, or boulders, it would be called a conglomerate. The fine-grained material first deposited as mud of varying degrees of fineness may go to form beds of clay, or shale, or eventually slate.
During the earlier history of the granite it has often occurred that chemical changes have been wrought in some of the felspars, mica, or hornblende ; the alkalies or lime have been taken from the felspars, and the lime and mag- nesia may have been taken from the other minerals, and these have united again in different combinations to form new minerals. This kind of rearrangement, with less opportunity for recomposition, becomes very active in the upper parts of the rock when exposed, and while the in- tractable quartz is left intact the alumina becomes hydrated and removed as clayey substance, and the alkalies, lime, magnesia, and iron are slowly and steadily removed in solution.
The processes of destruction thus briefly summarized take place, not only in granites, but in all other rocks, and
SANDSTONES AND GRITS 115
the materials carried off in solution and suspension are sooner or later deposited elsewhere. Thus the ultimate source of the beds of limestone in their many manifesta- tions is traceable to the minerals of igneous rocks.
In the following chapters we shall deal first with the sandstones and rocks of their kind, and later with the limestones and the derivatives of clay.
SANDSTONES, GRITS, CONGLOMERATES AND BRECCIAS.
It has already been indicated that these rocks have been formed from the degradation of pre-existing rock masses, and it is mainly due to the sorting action of moving water — rain, streams, and marine currents — that we now find in one place coarse breccia or conglomerates, in another grits or fine sandstones.
Since the agencies which effect the breaking up and transport of rock material have been active from the earliest epochs, we naturally find representatives of all these kinds of derivative bedded deposits in every geologi- cal system from the earliest to the present day ; and then, as now, we find that at any particular instant of time coarse sands were being laid down over one area, fine sands on another, conglomerates in a third, and so on, precisely as at this moment we have a great bank of coarse shingle — a potential conglomerate — in the Chesil Bank of the Dorset coast ; broad sandy stretches — potential sandstones — off the eastern counties ; or the coarse boulders of some of our rocky shores contrasting with the muddy flats of the Wash or the Thames. Moreover, since the degree of coarseness of any of these deposits is always dependent upon the velocity of the moving water, and as from time to time this velocity has varied, we find the coarse sands giving place to fine muds, clays, and shales, over any one area, and these changes were often repeated many times.
ii6 GEOLOGY OF BUILDING STONES
Bedding of Sandstones. — In common with all strati- fied deposits, the sandstones occur in lenticular beds — that is to say, sooner or later the beds thin out, and finally disappear in every direction. Occasionally great masses of sandstone, hundreds of feet thick, are split up into beds by mere partings or planes of separation, the individual strata ranging in thickness from a few inches to 30 or 40 feet ; in other cases the beds of sandstone are separated by layers of shale or by clay seams. The finer-grained sandstones — those deposited in gently moving waters — are usually more regularly and evenly bedded than the coarse varieties ; and when mica is abundant in such rocks, the flakes almost invariably tend to lie with their flat faces in the bedding plane ; and since there has been a sort of periodicity in the change of velocity of the water move- ment, the finer flakes have tended to settle down in the quieter phases, in this way producing the planes of ready fissility seen in certain flagstones and in a more marked degree in the more readily splitting tilestones.
Upon the bedding planes of the medium-grained and finer sandstones it is a common occurrence to find ripple marks preserved with great fidelity, along with rain-pits caused by primaeval showers, footprints of birds and beasts, and the impressions of plants. Very character- istic of many sandstones is diagonal bedding (current bedding or false bedding), due to constant change of direc- tion in the stream or current (or wind) which deposited the sand (Fig. 4). Sandstones and grits are on the whole more prone to exhibit diagonal or current bedding