Open-Air Studies. An introduction to geology out-of-doors ... With ... illustrations, etc

Introduction

TO GEOLOGY OUT-OF-DOORS standard works in geology. By the same Author. Second Edition, Revised. 10s. 6d. PRACTICAL GEOLOGY (Aids in) With a Section on Fossils. By Prof. GRENVILLE A. J. COLE, F.G.S. With Numerous Illustrations. " Professor Cole treats of the examination of minerals and rocks in a way that has never been attempted before. . . . DESERVING of the highest praise. Here, indeed, are 'Aids' innumerable and invaluable. All the directions are given with the utmost clearness and precision. Professor Cole is not only an accomplished Petrologist, he is evidently also a thoroughly sympathetic teacher, and seems to know intuitively what are stumbling-blocks to learners— a rare and priceless quality." — Athenæum. Demy 8vo, handsome cloth, 18s. Physical Geology and Palæontology. By HARRY GOVIER SEELEY, F.R.S, PKOFESSOR OF GEOGRAPHY IN KINO'S COLLEGE, LONDON.

With Frontispiece in Chromo-Lithography, and Illustrations. "So] much that is not accessible in other works is presented in this volume, that no Student of Geology can afford to be without it." — American Journal of Engineering. Demy 8vo, handsome cloth, 34s. Stratigraphical Geology and Palæon tology. By ROBERT ETHERIDGE, F.R.S., OF THE NATURAL HIST. DEPARTMENT, BRITISH MUSEUM, LATE PALÆONTOLOGIST TO THE GEOLOGICAL SURVEY OF GREAT BRITAIN, PAST-PRESIDENT OF THE GEOLOGICAL SOCIETY, ETC. With Map, Numerous Tables, and Thirty-six Plates. " No such compendium of geological knowledge has ever been brought together before." — Westminster Review. LONDON : CHARLES GRIFFIN AND CO., Ltd., Exeter Street, Strand. CWM-GLAS, PASS OF LLANBERIS. In the background are the aretes of Snowdon ; the wall between the upper and lower cwms lies below, with the great roc he moutounee in, the centre of the lower cwm. Moraine-blocks strew the foreground. Frontispiece.] s Photographed by Mr. J. J. Cole, F.R.A. OPEN-AIR STUDIES: AN

INTRODUCTION.

TO GEOLOGY OUT-OF-DOORS. BY GRENVILLE A. J. COL^, M.R.I.A., F.G.S., PROFESSOR OF GEOLOGY IN THls ROYAL COLLEGE OF SCIENCE FOR IRELAND; AUTHOR OF "AIDS IN PRACTICAL GEOLOGY" AND "THE GYPSY ROAD." OTttf) tfull=page -tllnatrattonB from P^otograpfje ant 5 tgura in tirje 2Eext. LONDON: CHARLES GRIFFIN AND COMPANY, Limited, EXETER STREET, STRAND 1895. [All rights reserved.] Printed by BalLantyne, Hanson & Co At the Balkwtyne Press TO RICHARD WHATELY DICKINSON This little book was in my mind all the time that ■we were studying together. So I dedicate it to you, and to all those who like seeing things for themselves in the open air. GRENVILLE A. J. COLE.

Preface

Now that the elements of chemistry and physics are at last taking their place in the fundamental courses of self respecting schools, we may fairly hope that a large number of persons will grow up with an intelligent interest in the world around them. In the first chapter of this book, I have gone lightly over some of the ground covered by such earlier lessons ; and I take heart from the fact that Mr. Small, in his excellent little work, " The Earth," has felt it incumbent upon him to make the same reminders. I then ask the reader to step boldly forth into the outer air. In our walks abroad, we may be struck by this or that detail, seemingly trivial in itself, which finally leads up to some one of the vexed problems of the globe. Even in so simple a book as this, I have not spared references to sources of infor mation, abroad as well as at home ; for one of the great incen tives in scientific life is the knowledge that in all countries our fellow-observers are ready, and that our friends are con stant in their aid.

I have tried to deal in a sufficiently wide spirit with the special districts that are referred to, so that the observations may be transferred and applied to the im mediate surroundings of the reader, or to the scenery of his summer holidays. Now-a-days, when cycling is so frequent and so free a means of travel, the geography and geology of nearer Europe have become keen realities to many of us, and almost all the references to landscapes in these chapters are the outcome of journeys on the road. I have wilfully mingled metric and English measurements throughout the book, since it seems that in this way alone we may hope to render popular the refined system of our neigh bours, a system too often restricted to purely technical works. Maps, moreover, form an important part of the equipment of an observer in the field ; and outside our islands we soon

PREFACE.

VIII learn to think in kilometres per centimetre when studying their various scales. The great aim of this little book is to keep in view the fact that geology, like true zoology and true botany, is a study of the open air. To dissect a rabbit in the laboratory is a more intellectual exercise than to shoot him as he runs across the sand-hills ; but the object of such study, after all, is the understanding of the rabbit when alive. The average sportsman, with a careful eye for the habits of an animal, may at times go nearer towards this end than the average student at his microscope ; and, similarly, it is only by a combination of two types of observation that a sound geologist can be made. Almost all great progress in natural know ledge has thus been made by those who have seen and travelled, by those, in fact, who have studied in Nature's roofless school.

Any person can now put himself in communication with his local field-club, in whatever quarter he may settle, and can thus receive the guidance of specialists in almost every county of our islands. For help in the many illustrations, which have been kept before me or which have been actually reproduced in these pages, I am indebted largely to active members of the Geologists' Association — to Dr. Tempest-Anderson, Prof. Johnston-Lavis, Mr. Henry Preston, Mr. E. P. Ridley, and Mr. F. Woolnough ; while the work of that true artist, Mr. Welch of Belfast, has added greatly to any interest which the book may now possess. I can only give my thanks for such good-fellowship and assistance, and may perhaps hope in turn to be of some service to my friends. It is especially pleasant to me to include two plates from my Father's series of geological photographs. which formed perhaps my earliest introduction to the study of geology out-of-doors.

GRENVILLE A. J. COLE. Dublin, May 1895. TABLE OF CONTENTS CHAP. PAOE I. THE MATERIALS OF__THE EARTH .... I II. A MOUNTAIN-HOLLOAV ..... 28 III. DOWN THE VALLEY . . . . . 58 IV. ALONG THE SHORE . . . . . -87 V. ACROSS THE PLAINS . . . . . .121 VI. DEAD VOLCANOES . . . . . . I48 VII. A GRANITE HIGHLAND . . . . .189 VIII. THE ANNALS OF THE EARTH . . . . 2l6 IX. THE SURREY HILLS ...... 244 X. THE FOLDS OF THE MOUNTAINS . . . 283 INDEX 314 LIST OF ILLUSTRATIONS FULL-PAGE ILLUSTRATIONS Cavm-glas, Pass of Llanberis Frontispiece PLATE I. Waterfall forming an Alcove in Stratified Rocks, Glencar, Co. Sligo To face page 42 II. Snow-filled Cirque at the base of the Matterhorn, Switzerland ........ ,, 56 III. Stratified Sands and Gravels, Antrim ... ,,76 IV. The Sea breaking on a rocky coast, near Ballycastle, Co. Antrim ...... ,, 96 V. The Crater-rim of the Puy de Pariou, Auvergne . ,, 150 VI.

Columns at the Giant's Causeway, Co. Antrim, in Basaltic Lava-flow ,, 172 VII. Dyke of Dolerite, Quarry on Cave Hill, Belfast . „ 181 VIII. Columnar and massive Lava-flows at Pleaskin Head, Co. Antrim ,, 183 IX. Granite exposed on slope above the Kilkeel River, Mourne Mountains, Co. Down . . . . ,, 190 X. Fossil Shells in Pliocene Sands, Felixstowe, Suffolk . „ 220 XI. Contorted Upper Jurassic Strata, Stare Cove, Lulworth, Dorsetshire ,, 284 ILLUSTRATIONS IN TEXT FIG. PAGE 1. Pot-hole, Glenariff, Co. Antrim 31 2. Section illustrating the origin of Springs 59 3. Section showing principle of Artesian Springs and Wells, and origin of Bournes 60 4. Section of Gorge choked with Alluvium 75 5. Section showing a slope of 1 in 17 79 LIST OF ILLUSTRATIONS no. PAQE 6. Sections of Alpine valley and Scotch glens 83 7.

Old Lake-Terraces of the Salt-Lake area of Utah . . >39 8. The island of Vulcano in eruption 155 9. Small Cones thrown up on Etna 159 10. Section of a Volcano 161 11. Floating Scoriæ above the Submarine Eruption off Pantelleria . 163 12. Microscopic section of Obsidian 165 13. Microscopic section of Basalt 167 14. The Puy de Lassolas and the Puy de la Vache .... 176 15. Geological Map, showing Lava-flows descending from Puys in Auvergne . . 177 16. Granite Pinnacles, Mourne Mountains 193 17. Unconformable Junction, with Overlap and Overstep . . . 219 18. Internal cast of a Cerithium, Portland Stone .... 221 19. a. Pecten Beaver i ; b. Pecten cinctus; <*. Pecten islandicus . . 225 20. Trilobites 235 21. Section across the Surrey Hills 249 22. Section illustrating Variation of Width of Outcrop . . . 253 23.

Sand with Spicules of Siliceous Sponges, Hythe Beds, Surrey . 261 24. Ancyloceras gigas .......... 269 25. Acanthoceras rothomagense . . . . . . . .271 26. Actinocaniax plenus ......... 273 27. Inoceramus Cuvieri .......... 274 28. Section illustrating Fan-Structure in a Mountain -Chain . 284 29. Section illustrating Recumbent Folds 284 30. Section across a Fault 285 31. Plan of a Fault in inclined Strata 286 32. Ridges and Valleys formed by Folds in the Jura Mountains . 291 33. Recumbent Synclinal and Folding on the Windgiille . .

Chapter I Thb Materials Of The Earth

When we were children, as soon as we could think and ask questions, most of us tried to find out two important matters — how to make things, and what things are made of. The little boy who cut open his drum to see where the sound came from was not so entirely foolish as is sometimes represented, for he at least made an interesting and valuable experiment. He learnt in this way that a drum was best constructed by stretching two thin skins opposite to one another, and shutting in a quantity of air between them. He was not satisfied with the mere showy outsides of things, or with the royal arms of Great Britain and Ireland painted on the wooden barrel of the drum. He felt that the sound, which was the great feature of the instrument, must have a cause ; and he set to work to find out something about it for himself.

Probably he only got as far as seeing how the drum was made, and had to ask his father for the reason of this particular arrangement. But that is what every one has to do, and why no great discovery is likely to be of use to us unless we know what other people have previously dis covered. In the same way, we cannot write a passably useful book unless we are near the 50,000 volumes of a well-stocked public library. Our knowledge in this world does not go on by jumps and bounds, but by a study of what others have seen and done, and of the records of their failure or success. Even if, after a long series of experiments, we have to leave our old drums damaged and cut open, we may soon have wit enough to make newer and better ones for the service of ourselves and our descendants. A In early times, it was most important that man should find out how to make things.

He had to build shelters against the rain ; he had to find weapons with which to combat the wild beasts ; he had to carve out gourds into vessels in which liquids could be carried. But all these things were by no means easy, and required a good deal of observation. Nature was already doing many wonderful things in the great world round about, and it was well to watch, and even to imitate, these closely. After all, what ever was required by man had to be made out of natural materials. One result of this struggle with difficulties was that early man took to studying the stones beneath his feet. Some of his earliest instruments were made of stone ; and we know that many savage tribes have only given up the use of stone axes and stone arrow-heads since the arrival of European traders on their coasts.

Primitive peoples would soon observe that some kinds of stone break more easily than others, that some kinds can be chipped in almost any direction so as to give a sharp cutting edge, and that some, again, are far superior in hardness or durability to the rest. It became important to search among the rocks for materials suited to each special purpose ; and thus began, perhaps, the first researches into the structure of the world around us. When native, metals were discovered, and when, moreover, it was found that valuable substances could be extracted by heat from stones utterly unlike them in appearance, the study of the earth became a much more complicated matter ; and at last a few thoughtful persons took to inquiring how such-and-such materials, used by man in the manufacture of common things, came themselves to be made in the heart of the old solid earth.

Then began the science of Geology, the systematic study of the way in which the world is built up, and of the natural changes which go on within it and upon it. Even the earliest men must have seen that the earth changed around them. Floods came down, and covered smiling landscapes with sand and stones from distant hills ; volcanoes broke out, and heaped ashes and lava upon the' surface ; the sea worked against the land, and washed the coast steadily away ; while in other places tawny rivers wandered through the plains and thrust their burden of mud in long banks far to seaward. Thus the earth changed its surface, growing at one place, decaying away at another ; and men began to realise that it had a history, beside which their own lives were short indeed. And yet it is only in the last hundred years or so that we have been able to read this history at all clearly, and even now we understand only that end of it which lies nearest to ourselves.

Before we go out and try to read a page or two of this history, we must learn something of the materials with which we have to deal. Of what, then, is the earth actually made ? As a matter of fact, we know only a very small part of the vast globe on which we live. The centre of the earth lies 4000 miles beneath us, and our mines and borings pene trate the great mass to a depth of only about a mile. But, as we shall see later, the outer layers of the earth have become wrinkled and folded, so that rocks which once lay far below the surface have been brought up within our reach. Hence in some places, immediately under the loose soil, we may find materials which were formerly ten or fifteen miles lower down ; and this enables us to say that we have some idea of the constitution of the globe to a depth of fifteen miles from the surface.

It is not much, this mere outer shell, fifteen miles thick all round the earth ; but it is all we shall have to deal with, and we speak of this accessible region as the crust of the earth. This solid crust is made of rocks. Any workman in a quarry will tell us, for instance, that granite is a rock, or that sandstone is a rock. But, when we examine these two materials, we see that they are made up of distinct particles which themselves are worth inquiring into. The granite has clear little lumps, like glass, in it ; and dull white or pink bodies, which seem to break across in a regular manner, with smooth or step-like surfaces ; and soft flaky things which reflect the light brilliantly, and which can be pulled away, or dug out with a knife, in the form of little shining plates. The sandstone, on the other hand, is made of practically one kind of material throughout, in the form of a number of hard and somewhat rounded grains.

In each case we call these constituents, having characters of their own, the minerals which compose the rock. The granite consists of at least three distinct minerals, the sandstone of only one. Now, if we break up a rock, and select a number of fragments of any one kind of mineral, a chemist will show us that this mineral is itself made up of certain chemical substances, which he can separate and accurately determine. He will be able to find out in what proportions these chemical substances are present, and will thus give us an account of the chemical constitution of the mineral. But what about these various substances? They may be stated by our friendly chemist to be chemical compounds, and these can be further split up into their elements. Or, to take the simplest case, our mineral may possibly consist of merely one element.

Here we may remind ourselves of some of our chemical notions. We must go back, indeed, to the foundations of several sciences for the correct understanding of our rocks, and this first chapter is going to be a serious one, before we can fairly get out into the country and the open air. But we must remember that all the things with which we shall deal are quite natural, all come from the earth itself that stretches round us ; and those parts of chemistry and physics which are possibly a little dull to us seem so merely because we cannot handle or directly observe the things about which we are being told. If we could see the tiny " atoms " moving in the " molecule " of a substance, we should pro bably be as delighted and as fascinated as if we were sitting on a mountain-peak and watching the great balance of the stars.

A chemical element is a substance that cannot be split up, by any means at present in our power, into any other substances with different properties from those by which it is itself characterised. It is thus the simplest kind of material known to us. Gold, silver, copper, and iron are elements; sulphur is an element; so also is carbon, well known to us in its commonest form, the " black-lead " of our pencils. There are now about seventy elements known, and others, at present doubtful, are likely to be added to the list. The ancients regarded the earth as an element in itself, something existing as a foundation for other things, the remaining elements being air, fire, and water. What we now know as elements were often treated in earlier times as varied forms of the same fundamental material ; but all experiment has shown us up to the present that there are really, in this earth and throughout the mighty universe, a number of different kinds of matter.

To be quite simple in our conceptions, we might wish to say that all elements are made up of different arrangements of particles of one first formed matter ; but that would be merely guess-work, and at present we cannot get beyond our seventy elements. If we conceive the smallest particle of an element that can independently exist, we may call such a particle an atom. It will have a certain weight, and the atoms of different elements differ in their weights. But any quantity of matter that we can see, even with a microscope, must consist of enormous numbers of atoms. The atoms are said to be grouped together in molecules, or "little masses." Chemists regard the molecule of certain elements as consisting of only one atom ; in others, two or even six atoms may form the molecule. A molecule has been defined by Professor Tilden as "the smallest quantity which is able to take part in or result from a chemical change." 1 A chemical compound is built up of molecules, each of which consists of atoms of more than one kind.

In the simplest compounds the molecule consists of only two atoms, one atom of one element being combined with one of another. The elements have been shown to combine together in fixed proportions, forming compounds with dis tinctive properties. We may hence say that the molecule of a compound is the smallest particle into which the sub stance could be divided while yet retaining its peculiar chemical constitution. Further splitting might give us certain simpler compounds ; but ultimately the actual atoms, or groups of similar atoms, of each molecule would be set free ; in other words, the compound would separate into its elements. All this may seem beyond human observation ; but large quantities of molecules of a compound can be dealt with in our experiments, and large quantities of elementary molecules can be extracted by chemical processes from them.

Or, on the other hand, we can cause large quantities of elementary molecules of different kinds to combine together to form large quantities of molecules of a chemical compound ; and 1 "Introduction to the Study of Chemical Philosophy," 1876, p. 4. in either case, by weighing the quantity of each substance dealt with, we can find out in what proportions the elements are combined in any particular compound. We may now glance backwards, and define an atom as the smallest quantity of an element that can be driven out of or caused to enter into a molecule of a compound in the subtle process of destroying or producing any of the known chemical combinations of that element. Here we have got down as far as we can go in the chemical constitution of a mineral. Minerals are made of chemical substances, which are made of molecules ; these molecules consist of atoms either (a) of different kinds, in which case we are dealing with a com pound, or (b) of only one kind, in which case our substance is an element.

Now we may try to arrive at a definition of a mineral. Let us look at those which we have separated from the granite — three materials evidently requiring distinct names, evidently differing markedly in their physical characters. The clear little lumps are called Quartz, and do not tell us very much by their external appearance. They break across with irregular curving fractures, much as glass does ; and they have in this case no regular shape. But they are transparent, and also very hard ; for we cannot scratch them with a knife, and they, on the other hand, will scratch glass. Moreover, they will not soften and melt, as glass does, when held with a pair of forceps in the flame of a Bunsen burner — the gas-burner used in chemical laboratories or in ordinary gas-stoves. Clearly, they are not grains of glass.

If we give them to a chemist, he will find in them the two elements, Silicon and Oxygen, always in the proportion of one atom of silicon to two atoms of oxygen. Quartz is made, in fact, of the oxide of silicon, commonly called Silica. We may take any of these grains from granite rocks found anywhere in the world, and yet their chemical composition will be the same. The dull white or pink bodies are called Orthoclase, or Orthoclase Felspar ; these, as we have already noticed, have the property of breaking across regularly in certain direc tions — in two directions, at any rate — when struck with a hammer. This shows that their internal structure, the way in which the molecules are grouped together, differs from that of quartz. Moreover, we can see something of their external shape ; it is often regular, and, even on roughly broken surfaces of the rock, the larger orthoclases are seen to approach the form of small flat bricks.

Then, in turning about the rock-specimen in the hand, some of these ortho clases seem to be built up of two parts ; that is, one half of the orthoclase catches the light and gives a bright reflection, while the other half looks dull ; clearly the planes of fracture are differently sloped in the two parts of the mineral, and it has not a perfectly simple internal structure. A good knife will just scratch the orthoclase, if drawn firmly across it ; but orthoclase will scratch ordinary glass ; we thus learn that it is harder than glass, but not so hard as quartz. A little splinter of orthoclase can just be melted after holding it for some time in the flame of a Bunsen gas-burner. Our chemist tells us that this mineral is more complex than quartz. He finds in it Silicon, Oxygen, Aluminium, Potassium, and generally Sodium.

The proportion of these elements to one another is always the same, except in the case of the two last named. Sometimes there is only potas sium present, and no sodium ; sometimes sodium occurs, and then there is less potassium. We say that the potassium may be replaced in part by sodium without the principal charac teristics of the mineral being altered. In such cases it must be a matter of general consent and judgment as to whether we are to call the mineral by a new name when such a replacement occurs. The atoms of sodium are lighter than those of potassium ; but any difference in the weight of a given bulk of orthoclase, according as it contains potassium, or both potassium and sodium, would be very trifling. Cases often occur, moreover, where a lighter element replaces a heavier one, and yet the resulting variety of the mineral is actually heavier than the ordinary form.

The molecules con taining the lighter element must in such cases lie closer together, i.e., there must be more of them in a given space, than occurs when only molecules containing the heavier element are present. So we may note that the chemical composition of some minerals may vary slightly, varieties being set up which are grouped together under the common mineral name. A molecule of our typical orthoclase contains 2 atoms of potassium, 2 of aluminium, 6 of silicon, and 1 6 of oxygen. But in most specimens of orthoclase there are some molecules in which two atoms of sodium take the place of the two atoms of potas sium. When such molecules are shown by a chemical analysis to be numerous, we call the mineral variety Soda-Orthoclase. Our third mineral in the granite is called Mica — the plate like substance which is easily dug out with the knife, and which gives such a shining appearance to many specimens of the rock.

Some micas are pale and silvery ; others are quite dark, being bronze-coloured or black ; but they have many characters in common. Again and again their proper form can be seen ; their molecules are clearly capable of building up six-sided plates. This structure is so common that it can not be accidental ; and careful measures show that the angles of the hexagonal figure are kept the same in the same chemical variety. Such a structure is called a crystal ; and a little observation will show us that the orthoclase also is in the form of crystals, though they have probably not been able to grow to such perfection. The mica splits very easily in one direction, so that we can flake off thin flexible plates ; and even the blacker micas are no longer opaque when one of their thin plates is held up to the light.

The mineral is quite soft, and can often be scratched by the thumb-nail. The chemical composition of the micas is still more complex than that of the orthoclase felspar, but all common varieties contain Silicon, Oxygen, Aluminium, Potassium, Magnesium, Iron, and Hydrogen. The pale varieties con tain little iron and usually little magnesium ; the dark varieties are richer in these elements and poorer in silicon and aluminium. There may be a good deal of replacement of one element by another iu the mica series, the general external characters being retained ; and several different kinds or species of mica are recognised, and are known by distinct mineral names. If we are so fortunate as to find a granite with a number of small hollows in it, perhaps half-an-inch or an inch across, such as occur in the famous rocks of the northern part of the Mourne Mountains, then we may see that all the minerals which we have discussed are capable of forming crystals, wherever freedom is given to them and when they are not too much squeezed together.

In such hollows we find the mica, and the orthoclase, and the quartz itself, delicately crystallised, beautifully and regularly shaped, so that persons looking at the handsome crystals in public collections often think that they have been artificially cut and polished. But you have only to go into the hills, and search in cracks and hollows for yourselves, to find that crystals occur naturally, and that they are, indeed, the form frequently adopted by natural chemical substances. And, further, the crystals of one compound usually differ from those of another ; the sides make different angles one with another, even if the forms are very much alike ; so that, by careful observation and measurement of the angles in various crystals, we can use the outer shape to help us in determining the nature of the mineral.