THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA RIVERSIDE Ex Libris C. K. OGDEN 1 OUTLINES OF FIELD-GEOLOGY. OUTLINES FIELD-GEOLOGY. BY ARCHIBALD GEIKIE, LL.D,, F.R.S., . I i of Great Britain and Ir ^ in the Univer " of the Geological j ur~jcy of THIRD EDITION : WITH NUMEROUS ILLUSTRATIONS. MACMILLAN AND CO. 1882. The Right of Translation Reserved. RICHARD CLAT & Sows, AD STREET HILL, LONDON. K.C. And at Runga V . SufoVt. PREFACE TO THE SECOND EDITION. Ax the request of the Lords of the Committee of Privy Council on Education I gave in the month of August, 1876, at South Kensington, two lectures upon vgeological maps and instruments of surveying. These \Mectures formed part of a series designed for teachers, and in illustration of the Loan Collection of Scientific \J % v. Instruments at that time exhibited. Treating ^the sub- ject allotted to me in what seemed likely to prove the . most useful manner, I dwelt more specially upon the A methods of observation requisite in ordinary field- N geology ; and endeavoured to show how by the practice fcj of these methods geological maps and sections, repre- x senting in condensed form the facts established by ^v field-work, could be constructed. The lectures were published in pamphlet form later in the autumn of the same year. vi PREFACE. A large impression having been sold and the work having been for some time out. of print, the publishers asked me to allow it to be reprinted in a more perma- nent shape. I delayed complying with this request until I could find leisure to revise and extend the lectures. I have now entirely recast them, and, dropping the original lecture form, have thrown the matter into chapters, with distinct headings. So great have been the additions that the little volume now issued may claim to be a new and independent work. It retains, however, the un- pretending elementary character of the original lectures. Aly aim has been to write primarily for that large and increasing body of readers who have made some general acquaintance with geology, but who, though much in- terested in the subject, find themselves helpless when they try to interpret the facts which they meet with in the field. The practical knowledge of which they feel the want is not indeed to be gained from books. It must be sought in quarries and ravines, by hillside and sea- shore. But hints regarding what should be looked for arid how to set about the search may not be without some usefulness. And these it is the object of the following pages to give. The young geologist into whose hands this little book PREFACE. vii may fall will therefore remember that it is not meant as a systematic text-book on geology. It presupposes him to have already read some such text-book, to have acquired a general knowledge of the scope of the science, and to have become in some measure familiar with the facts. Its purpose is to be suggestive to him, rather than didactic ; to put him in the way of intelli- gently observing for himself, rather than to present him with what has already been discovered by others. COLLEGE, EDINBURGH, February ', 1879. PREFACE TO THE THIRD EDITION. A FEW additions and verbal corrections have been made to this edition. The reader will find some portions of the work more fully treated in my Text-book of Geology. 28, JERMYN STREET, LONDON, October, 1882. CONTENTS. CHAP. I. INTRODUCTORY PART I OUT-OF-DOOR WORK. II. FIRST ESSAYS IN FIELD-WORK IO III. ACCOUTREMENT FOR THE FIELD l8 IV. GEOLOGICAL MAPS 31 V. PRELIMINARY TRAVERSES OR RECONNAISSANCES- CIRCUMSCRIBING CONDITIONS 39 VI. DETERMINATION OF ROCKS 48 VII. THE NATURE AND USE OF FOSSILS 7 VIII. THE TRACING OF GEOLOGICAL BOUNDARY LINES . 87 IX. THE UNRAVELLING OF GEOLOGICAL STRUCTURE DIP, STRIKE, OUTCROP 94 x CONTENTS. CHAP. PAGE X. THE UNRAVELLING OF GEOLOGICAL STRUCTURE FAULTS IIJ XI. THE CURVATURE OF ROCKS 13! XII. IGNEOUS ROCKS 14! XIII. SCHISTOSE ROCKS MINERAL VEINS 156 XIV. SURFACE GEOLOGY l66 PARTII.IN-DOOR WORK. XV. NATURE OF INDOOR WORK GEOLOGICAL SECTIONS 179 XVI. CHEMICAL TESTS IN THE IDENTIFICATION OF MINERALS AND ROCKS l88 XVII. MICROSCOPICAL INVESTIGATION 291 LIST OF ILLUSTRATIONS. PIG. PAGE 1. Geological hammer, compass-case, and belt ... 22 2. Clinometer 24 3. Maps showing the effect of incorrect topography in dis- torting geological lines 35 4. Some useful signs in geological surveying ... 38 5. Weathered crust, showing concentric zones of oxidation . 51 6. Dolerite (basalt) weathering spheroidally. North Queens- ferry 53 7. A piece of granite. Crystalline structure . . . 55 8. Piece of lava, showing crystals and steam-holes . . 56 9. Piece of conglomerate, showing the characteristic rounded water-worn aspect of the component parts of many fragmental rocks 58 10. Piece of volcanic tuff 59 11. Piece of coal, composed of matted stems of Sigillaria and Lepidodendron. Carmarthenshire (De la Beche) . 60 xii LIST OF ILLUSTRATIONS. FIG. PAGB 12. Outlines of mountains formed of stratified or sedimentary rocks. Rocky Mountains (Hayden's Report of Survey of Western Territories, 1874) 66 13. Outlines of a mountain formed of crystalline rock. Rocky Mountains (Hayden's Report of Survey of Western Territories, 1874) 68 14. Fossils standing in relief on a weathered surface of limestone 74 15. Ripple-marks in sandstone 77 1 6. Rain-prints in sandstone 78 17. Sun-cracked surface of red sandstone marked with foot- prints, Hildburghausen, Saxony .... 79 1 8. Limestone bored by lithodomous shells .... 80 19. Section of a buried land-surface (De la Beche) . . 81 20. Section of inclined carboniferous rocks, Joppa Quarry, near Edinburgh 82 21. Section to show the superficial covering of soil ; subsoil derived from the disintegration of the underlying rock ' ... 88 22. Section of a valley showing the outcrop of a junction of sandstone and shale marked by a line of springs . 90 23. Sinuous outcrops of horizontal strata depending on in- equalities of surface ....... 96 24. True dip concealed by superficial disturbance of the strata 97 25. Inclined strata appearing horizontal when exposed at a right angle to the dip . .- . . 98 LIST OF ILLUSTRATIONS. xiii FIG. PAGB 26. Map showing the data from which a complete geological tade . 102 27. Complete geological map ...... 107 28. Section of an overlap no 29. Overlap and unconformability, Mendip Hills (De la Beche) Ill 30. Portion of a geological section with an impossible un- conformability 113 31. Double unconformability at Cullen, Banffshire . . 115 32. Section of lias and new red sandstone and marl cut by faults near Watchet, Bristol Channel (De la Beche) . 119 33. Section of a fault 121 34. Fault marked by the rise of springs at the surface . 122 35. Geological section showing how a prominent feature at the surface may be caused by the outcrop of a hard rock intercalated among softer strata . . . 123 36. Section of fault with inverted beds on the down-throw side 125 37. Section of inclined strata 132 38. Section of inclined strata, showing that they form part of a large curve 132 39. Section of strata curved in an arch or anticline . . 133 40. Section of strata curved in a trough or syncline . . 133 41. Anticlines forming valleys ; synclines forming hills . 135 42. Curved limestone, Draughton, near Skipton . . 135 xiv LIST OF ILLUSTRATIONS. FIG. PAGE 43. Shales contorted by a landslip 136 44. Section of what at the surface might be mistaken for a continuous vertical series of strata, shown to consist of numerous anticlinal and synclinal folds . . 137 45. Inverted contortions . f + . . . 138 46. Section of the grand inversions of strata in the Glamisch Mountains, Eastern Alps 138 47. Cleavage of curved strata 140 48. Section of a portion of the earth's crust broken by two disiscations whereby three different masses of strati- fied rock have been brought into juxtaposition, and with two masses of intrusive igneous rock which have risen along the lines of fracture . . . . 144 49. Section of interbedded igneous rocks, Linlithgowshire . 145 50. Upper service of an intrusive igneous sheet with over- lying shale 146 51. View of the Island of Staffa and Fingal's Cave, show- ing amorphous and columnar basalt resting upon tuff 145 52. Section of a dyke rising through a small fault and send- ing out a vein and an intrusive sheet or bed into the surrounding shales and ironstones .... 147 53. Upper surface of an interbedded igneous sheet with sedimentary strata lying upon it . ,. . . 151 54. Steam-holes in lava drawn out in the direction of the flow of the mass and becoming more and more flat- tened till on the line they are compressed into mere streaks and give a fissile structure to the rock . . 152 LIST OF ILLUSTRATIONS. xv FIG. PAGE 55. Cracks in an old red sandstone lava which have been filled in with sandstone from above. Coast of Kin- cardineshire 153 56. Granite veins. Cornwall (De la Beche) . . . 155 57. Foliation of mica-schist and grit-bands, coincident with bedding , ... 162 58. Map of part of the mining district of Gwennap, Corn- wall 164 59. Cliff cut into buttresses and recesses by means of the vertical joints of the rocks 167 60. Plan of a peat-moss which has filled up a former shallow lake, except one small patch of water . . . 168 61. Section of a peat-moss made in the excavation of the peat for fuel 169 62. River terraces . . . . . . . . 170 63. Ice-worn hummocks of rock 175 64. Striated stone from the boulder-clay . . . . 177 65. Illustrations of geological section drawing . . . 184 66. Stages in the construction of a geological section . . 186 OUTLINES OF FIELD-GEOLOGY. CHAPTER I. INTRODUCTORY. To those who are fond of the country and of long rambles there, geology offers many attractions. Few men are so unobservant as not to be struck now and then by at least the more salient features of a landscape. Even in a flat featureless country, the endless and ap- parently capricious curvings of the sluggish streams may occasionally suggest the question why such serpentine courses should ever have been chosen. But where the ground rises into undulations, and breaks out into hills and crags ; still more, where it towers into rugged moun- tains, cleft by precipices down which the torrents are ever pouring, and by ravines in the depths of which the hoarse streams ceaselessly murmur, one can hardly escape the natural curiosity to know something about these singular aspects of the landscapes, when and how they arose, and why they should be precisely as they are. For the day is now happily past when the sterner features of the land 2 OUTLINES OF FIELD-GEOLOGY. [CHAP. awakened only a feeling of horror; when they were styled hideous, and unsightly; when they were never visited save under the necessities of travel, and were always left behind with a sense of relief. Relics of these feelings survive to us in such phrases as wild, savage, uncouth, with which we still describe that mountain-world, once an object of awe and fear, now the centre to which a yearly increasing crowd of visitors repairs for some of the purest pleasures and most healthful recreation which this world has to afford. With the growth of an appreciation of natural scenery in all its forms, rugged as well as gentle, there has arisen also a desire to know on what causes these diversities of outline depend. We are not now content as our fathers were to accept the present aspect of a country as that which it has worn from the beginning of time. And even if no intelligible answer can be given to them, the questions I have referred to will ever and anon force themselves on our attention. Nor is it only the larger and more impressive features of the landscape which suggest such inquiries. A boul- derstone perched on some slope or on the edge of a crag, seemingly so perilously poised, that a mere push with the hand should send it rolling into the plain below, will raise in our minds the questions why the block should have stopped where it is, whence did it come, how was it carried, and what arrested it there ? On either side of a soft, well-cultivated valley we may perhaps detect, peeping out at intervals from among the wood- land orchards and cottages, a strong rib of rock, form- ing everywhere a marked feature, with its grey, lichen- crusted face, deep fern-hung shadows and tufts of sweet briar, honeysuckle, or bramble. Its regularity of level I.] INTRODUCTORY. 3 suggests at a distance some artificial land-mark a road, or fence, or some ruined rampart. Yet we see on nearer inspection that it must be a natural feature, and we ask ourselves why it should have arisen on the face of those green declivities, and why its course on the one side of the valley should be so exactly repeated on the other. Perchance a prominent mound rises from the general level of a plain, so singularly as to have attracted notice from the earliest times, and to have become the origin of a local myth. It seems too large to be due to man's operations, even if any intelligible reason could be assigned for his having heaped it together. On the supposition that it is natural, however, how are we to account for its existence ? What agent could have lifted its materials and piled them up into that solitary cone ? Again, the merest fragment of stone picked up on any everyday ramble may furnish questions, for the due an- swering of which many years of profitable study might be needed. A piece of limestone, for example, may show us on its fractured surface abundant fragments of corals and shells. With a little inquiry it may not be difficult to ascertain the source of the stone, and to visit the rock ; .n place. We may there find a thick bed of limestone, crowded throughout with similar organisms, and extending for miles across the country. It would need but a slender acquaintance with modern science to make us feel assured that this limestone must represent an old sea-bottom once thickly covered with living things. We might muse on the strange vicissitudes of nature, wherein the busy floor of a former ocean should have been changed into a land "made blithe with plough and harrow," and might ask ourselves how and when these revolutions were effected. B 2 4 OUTLINES OF FIELD-GEOLOGY. [CHAP. In the course of another walk we might stumble upon a bit of stone made up of rounded pebbles cemented together, as if a handful of gravel from some river-side had been hardened into stone. Could we trace this fragment also to its original locality, we should find it to have formed part of a larger bed or mass of what is called conglo- merate or puddingstone, and we should recognize still more the exact resemblance of the constituent stones of this rock to the shingle of a sea-shore, or the gravel of a river-bed. We could not for a moment doubt that the rock must be merely so much compacted water-worn gravel. But where lay the water by which these stones were rounded and polished ? Was it the sea, or a lake, or a river? What was the aspect of the country then, and through what cycle of change has it past to reach its present condition ? Thus even to one who knows no geology, the problems of the science are presented at every turn and in every country ramble. When, however, some acquaintance with this science has been gained, the number of questions which arise for solution rapidly increases, and with their growth there augments also the power of answering them, or at least the pleasure of seeking for their solution. The observer, as he finds his knowledge and consequently his confidence enlarged, discovers on the one hand that facts which he took for granted, and which never raised in his mind any question or difficulty, now demand some ex- planation, and, on the other, that he has to disabuse him- self of many prejudices or notions which grew up in his mind, he cannot tell when or how. For example, it never used to occur to him that there was anything especially deserving of notice in the fact, that the stones of an old i.] INTRODUCTORY. 5 building almost invariably have lost their sharp edge, and in many cases are crumbling and honey-combed. But he now observes these aspects, and derives from their study another pleasure to be added to the many which an in- teresting ruin yields to every one. He notes what kinds of stones decay most, on which aspect of the building the weathering is most advanced, and endeavours to ascertain on what circumstances the disintegration seems to have depended. For he recognizes that the walls of a building may be likened to the sides of a crag or precipice, and that in contemplating the progress of decay in a human edifice, he can learn not a little respecting the laws which govern the disintegration of the moun- tains. Again, he no doubt began with the common popular belief that the striking features of a landscape, notably its crags and ravines, are to be referred directly to the operation of earthquakes and former convulsions of nature. Slowly and perhaps with some difficulty he rids himself of the incubus of this prejudice. He re- fuses any longer to be bound by preconceived theory or explanation, but insists on being allowed to judge of each instance on its own merits and to reason upon it with reference to all its surroundings. If after having familiarized his eyes with the outward aspect and inner geological relations of his own district, the observer extends his journeys into other regions, he carries with him an added power of enjoyment in every country through which he may wander. He finds that an acquaintance with geology, far from blinding him to the softer beauties or wilder grandeur of a landscape, really quickens his perception of these charms. Practice enables him to take in at a glance the dominant features, and to 6 OUTLINES OF FIELD GEOLOGY. [CHAP. range the others in their orderly subordinate places, so that the harmony of the whole is seized, and the impres- sion which it makes is fixed upon the mind. If I may be allowed to make the comparison, it is with the apprecia- tion of scenery as with the cultivation of music. Most listeners of average education and intelligence thoroughly enjoy a sonata of Beethoven ; they listen to a harmonious variety of sound, and perhaps at the close awake almost out of dreamland. And yet, high as is their enjoyment, it can hardly equal that of the musician who recognizes, as movement succeeds movement, the skill and genius of the composer who could so vary and amplify some simple theme, and while seeming to abandon himself to a tumultuous torrent of sound could keep every portion of the work under the strictest rules of art, and with a breadth and harmony that bind the composition into one magnificent whole. Should the traveller find himself with leisure sufficient not merely to look at the scenery but to examine the rocks which form its groundwork, he will again find his experience at home stand him in good stead and give fresh interest to his journey. He will encounter other and often better illustrations of phenomenon with which he has already become practically familiar. He will perhaps meet with facts which throw a bright light on questions which had long puzzled him in his own country. Or he may see for the first time, and with a joy which he alone can experience, an example of some piece of geo- logical structure which he has known only from books, but which he now and for ever vividly realizes. In all this it is not needful that he should claim to be a geologist. He may not consider his observations I.] INTRODUCTORY. 7 worthy of attention from professed geologists, or he may have neither time nor inclination to publish them. But none the less does he enjoy the refreshment, alike bodily and mental, which geological work in the field brings with it. Should he, however, deem it proper to give the world the benefit of his labours, he may have the satisfaction of adding to the sum of knowledge, and of eliciting the thanks of geologists who will gladly admit him of their number. More especially should he be encouraged to publish his observations when they relate to unvisited or little known regions, or to tracts where he has enjoyed exceptional advantages for studying geological phenomena. But how is this geological experience to be acquired? How often do we meet with men who have read ex- tensively in geology, yet if they are set down among the rocks find themselves hopelessly adrift, and after some despairing efforts to recognize in nature what seems so clear in the diagrams of a text-book, give up the pursuit in disgust. On the other hand, how constantly are men to " be encountered who labour under the delusion that nature is as easily read as the manual whose pages they have so often skimmed over, and who proceed at once to quarry, hillside, or mountain, and explain its geological features with much more confidence than those would pretend to do who have made the subject their prolonged study. It is not from books alone that a man can acquire that practical acquaintance with geology which will minister so much to his elevation and enjoyment. He must betake himself to nature from the first. His lessons in the field should accompany his lessons from the text-book or lecture- room. In many cases he must 8 OUTLINES OF FIELD-GEOLOGY. [CHAP. grope his way without guide or assistance. His progress will be slow, but in the end he may find that it has been none the less sure and pleasant, and that, through this very tardiness of his advance, he has been compelled to master thoroughly every foothold of the way. The following chapters are offered for his help. They are Tfc>*4UStand in .the place of a systematic text-book, of which he will find still constant need. But still less are they to be looked at as in any way a substitute for prac- tical observation in the field. Their aim is to point out how observations may be made, what kinds of data should be looked for, what sort of evidence should be sought to establish a conclusion, and what deductions may be drawn from particular facts. In short, they are to be regarded as sign-posts pointing out some of the high-ways and bye-ways of geological inquiry, but leaving the reader to perform the journey in his own fashion. Their object will be fully realized if they induce him to find so much interest in the pursuit as to adopt it as a frequent solace for his leisure hours. But they are so arranged that it is hoped they may not be found without service to young geologists, who, whether at home or abroad, would fain devote themselves with energy to the task of geological investigation. The term Field-Geology, which I have selected as expressive of my subject, points then, to practical work in the open field, as distinguished from the researches which may be carried on in the library or laboratory. I wish to describe some of the methods by which a geologist obtains his information regarding the nature, position, arrangement, and history of the rocks of a country. Such practical observation evidently underlies I.] INTRODUCTORY. 9 all solid research in geology. He who would pursue the theoretical parts of the science must either himself lay a foundation in good thorough field-work, or take advan- tage of the foundation which has, in this respect, been laid for him by others. Field-geology may be pursued with various aims and in various ways. To some men it is little more than another name for holiday-making in the country fresh air, healthy exercise, new or old charms of scenery, and a bag full of " specimens " to attest the scientific nature of the work. To others it is the solace and delight of busy lives, furnishing them not only with bright intervals of escape to the country, but with materials for much profitable thought and study when the ordinary duties and cares of life confine them to their work in town. To other men, again, it is itself the main occupation of life, whether they cultivate it for its own sake, or with a view to the economic applications of which it is susceptible. There are few countries or districts where field-geology may not be cultivated, and where its healthful influence as an educational instrument may not be tested. A few days of intelligently guided observation in the field are worth far more to a pupil than many weeks of lectures and reading. But we seldom hear of such practical instruction, mainly because the teachers never received it, and have not had time, inclination, or opportunity to develop it for themselves. PART I. OUT-OF-DOOR WORK. CHAPTER II. FIRST ESSAYS IN FIELD-WORK. THE direction in which the first essays of the observer in the field should be made, must depend mainly upon the nature of the district in which he finds himself situated. Under the most unfavourable circumstances, as for instance in a wide cultivated plain, with not a single quarry or natural opening to show even the nature of the formations underneath, he may nevertheless discover something to engage his attention. Thus, he may find useful employment in watching the operations of the streams which flow sluggishly through his neighbourhood, their meanderings and the efforts they make to straighten their courses, their varying quantity of mud, the effects of floods, the evidence of successive deposits, and height- ening of the flood -plain. But it will seldom happen that he cannot in some way gain access to the geological formations below the surface, and even in a flat and featureless region obtain a series of facts which will CH. n.] FIRST ESSAYS IN FIELD-WORK, II enable him to reason as to the history of the region, and to decide whether the plain has been formed by the stream, or on the floor of some ancient lake, or perchance on the bed of the sea. Where, however, numerous openings, either natural or artificial, expose the strata underneath, the observer need be at no loss for abundant material for profitable field work. Should some of these strata be eminently fossili- ferous, that is, crowded with the remains of once living plants or animals, they will almost certainly attract his earliest attention. Probably in the majority of cases men have been led to the study of geology by first becoming interested in the organic remains which they could collect for themselves, carry home as " specimens," and afterwards thoughtfully question as to their structure and history. No doubt the mere gathering of the fossils is the first and final achievement of a very large pro- portion of enthusiastic beginners. Even, however, if the pursuit has had no other advantage than that of affording ample exercise in the open air, it is perhaps not less beneficial than many of the time honoured forms of out-of-door recreation. But a man may gain much more than healthful amuse- ment from fossil-hunting. He begins, let us suppose, by trying to get hold of as many varieties, and as perfect specimens as he can find by the most patient search. But the mere pleasure of the pursuit soon begets a desire to know more about the fossils. If they are plants, the collector strives to ascertain their names, and may be content perhaps if he can write upon them their proper Latin or Greek appellations. Possessed, however, of a real desire for knowledge, he seeks to ascertain what are their 12 OUTLINES OF FIELD-GEOLOGY. [CHAP. affinities with the living vegetation of to-day. By reading, by visiting museums, and by careful observation along the hedgerows or in botanic gardens, he endeavours to realize what the leaves and stems, which he finds in the solid stone, really were when they waved bright and green in the air long ages ago. The information he can glean as to their probable botanical grade and habit, leads him to re-examine, with greater care, the circumstances under which they lie in the rock. He finds, perhaps, that they occur more particularly in one stratum, which we shall suppose consists of thin leaves or laminae of a kind of hardened clay. It is on splitting up thesa laminae that he unfolds the fossil plants. Each layer seems entirely covered with impressions of leaves, stems, fruits, or other parts of the ancient vegetation ; but the fossils are all fragmentary, though well preserved. They remind him of the sheddings of trees after some early autumnal frost ; the fine layers of hardened silt, on which they lie, recall the laminae of mud which he has observed in the bottom of a pond or dried-up pool ; and in the end, he concludes with some confidence that his fossil-bearing stratum was once the floor of some inland sheet of water, into which the leaves of the neighbouring woodlands were periodically shed. If he has ascertained that the plants are more nearly allied to those of a warmer re- gion than the vegetation now flourishing in the locality, he allows himself to speculate on the probability that a warmer climate once prevailed in his own country. The remains of animals, however, are immensely more abundant among the rocks than those of plants. The observer is much more likely, therefore, to begin by lighting upon some stratum full of shells, crinoids, corals, ii.] FIRST ESSAYS IN FIELD-WORK. 13 or even with bones of fishes, and perhaps of reptiles. If he is not satisfied merely with forming a collection of these remains and having them rightly named, but wishes to learn what they have to tell him about ancient types of life and old conditions of physical geography, he addresses himself to the task by endeavouring to find the nearest analogies in the living world to the fossil forms which he has disinterred from the rocks. Patiently he tries to reconstruct the skeleton of which he has found the scattered bones. He learns to recognize the fragment of a shell or other fossil, and can assign it to its place in the complete organism. While the structure and zoological relations of the fossils afford him inexhaustible stores of employment, he cannot shut his eyes to the circumstances in which these fossils occur, and to the light which they cast on the history of the rocks. Corals, crinoids, and marine types of molluscan life bring before him an old sea-floor, and though the locality where his leisure hours are thus sedulously spent may now lie far in the heart of a country, with venerable trees and hedgerows, old farmsteads and roads, all bearing witness to the peaceful cultivation of centuries, the sight of that rock with its crowded fossils is as sure evidence of the former presence of the sea over the whole landscape, as if he heard there even now the murmur of the waves. But the observer's lot may be cast in a district where no fossils are to be found. There may be nothing in the rocks themselves to attract notice, nothing likely to inspire a taste for geology or to furnish nutriment for a taste already existent. It is remarkable, however, in what apparently unfavourable circumstances an appetite for I 4 OUTLINES OF FIELD-GEOLOGY. [CHAP. scientific pursuits can not only exist but flourish. Let us suppose that the district in question consists of stra- tified rocks, like sandstones and shales, and that these strata are exposed to view in numerous quarries and natural sections. The varying composition of the beds, their order of succession, their changes in character as they are traced over the country, their influence upon the contour of the ground, the glimpses they afford of an ancient geography very different from that of the district to-day, and the manner in which they have been tilted up, curved, and broken since the time of their original formation these, and a thousand other parti- culars, will eventually give even barren and seemingly repulsive rocks a charm which the richly fossiliferous deposits of the observer's later experience may never possess. If, on the other hand, the rocks are crystalline granites, schists, and other similar masses, or basalts, tuffs, and other volcanic accumulations, the geologist, who begins work among them will almost of necessity devote himsalf to the mineralogical and structural side of the science. He may be first attracted by pretty minerals, sparkling felspars, well crystallized and variously coloured quartzes, glittering micas, and many more. And doubtless the temptation to collect them, if it once arises within him, will not be likely to diminish, so long as his taste for geological pursuits lasts, and as he finds himself face to face with the minerals in the field. Pursued not as the hobby of a collector, but as an important branch of the sciences which deal with the architecture of the globe, mineralogy becomes a singularly fascinating study. I shall have occasion in later chapters to allude to some of its attractions. Should the observer be led from the ii.j FIRST ESSAYS IN FIELD-WORK. 1-5 minerals to the investigation of the rocks among which they lie, he will find himself in presence of some of the most interesting problems in geology. Some of these crystalline rocks are amongst the oldest of the globe; their origin is linked with the earth's early history, they are the witnesses of the power of that internal heat which has played so notable a part in the growth of the solid land. As a rule, too, the districts where they occur are more rugged than those which the fossiliferous formations overspread : hence they present everywhere crags, knobs and bosses of rock, as well as the more continuous sec- tions of water-courses. By these frequent exposures the successive bands of rock can be traced across the dis- trict ; their variations in breadth, in composition and in mineral contents can be followed ; and their intercalations, curvatures, fractures, and veins, can be unravelled, so as to reveal, more or less clearly, the structural plan of the whole region. In most places, save on the face of precipices and steep declivities, the rocks which form the framework of a country are more or less concealed by various super- ficial accumulations. Even should he never set himself to the study of the underlying formations, the observer may find ample scope for inquiry in these upper deposits. In one region he will encounter thick masses of earth or loam, containing here and there the bones of long extinct mammals. In another quarter he may meet with sheets of gravel, perched on the sides of valleys high above the present streams, yet evidently themselves of fluviatile origin, and containing scattered rude implements of human workmanship. In yet a third locality he will find a mass of clay, stuck full of stones with their surfaces 16 OUTLINES OF FIELD-GEOLOGY. [CHAP. polished and scratched like the rocks below a Swiss glacier, and he will learn that these striated stones and the clay containing them have once likewise been under a sheet of ice. In short he will soon perceive that in every one of the many varieties of superficial deposits there is a story to be made out, and that it is worth his while to decipher it. Lastly, it may chance that the beginner is so situated as to be able to watch the actual visible progress of geo- logical changes. His home may be by the margin of a river liable to occasional floods, and always bearing onward past him its burden of mud from the distant hills. No better training in geological observation could he desire than that which is supplied by a careful and methodical study of the operations of a river. Its times of flood and of low water, the proportion of mineral sub- stances in its water from month to month, the way in which the sediment is disposed of, the action of the river on its banks, here cutting down and there heaping up, the relation of the form of the channel to the rocks through which it has been cut, now a ravine, now a waterfall, here a rapid, there a lake-like reach these and many other points in the physics of a river furnish endless material of ever fresh interest. The stream has its moods like a living thing; no two years of its operations are exactly alike, and it seems always to have surprises in store for us, though we have watched it for years and are familiar with it under every aspect. Even more fortunate is the observer whose dwelling lies not only near a river but within reach of the sea. Even if the shore be low and sandy, he can watch the breakers as they come tumbling in upon the beach, and II.] FIRST ESSAYS IN FIELD-WORK. 17 mark how the colour of the water changes as it drags back the sand in its recoil. The sight of this ceaseless grinding impresses him, as hardly any other can do so well, with the way in which the boulders and gravel are reduced to the state of sand and spread over the sea-floor, there to lay the foundations of the land of future ages. But should the coast be rocky, he may congratulate him- self on having been placed in a kind of geological para- dise. Hardly anywhere else will he meet with the same facilities for observation. The beach serves as a platform on which the rocks are exposed for his study, and which is swept clean for him twice every day by the tides. He may devote himself to the investigation of the rocks them- selves, their contents and history, or he may observe the way in which they yield to the attacks made upon them by the sea on one side and by the air, rain, frost, and springs on the other. We may conclude, therefore, that there must be very few parts of the world where some kind of field-geology cannot be pursued. If the beginner who has read enough in the science to make him desirous of becoming himself an observer, finds it hopelessly impossible to extract any information or interest from his surroundings, he will probably be right in suspecting that the fault lies in himself, and not in them. Perhaps the chapters which follow may suggest some method of overcoming his difficulty. CHAPTER III. ACCOUTREMENT FOR THE FIELD. THE nature and extent of a geologist's accoutrements will, of course, be regulated by the kind of work he pro- poses to undertake, and the character of the rocks among which he is to be engaged. If his object be the collec- tion of specimens of minerals, rocks or fossils, he will require one sort of apparatus : if it be the study of the geological structure of the region, he will provide himself with another sort. It must be distinctly understood at the outset, that the popular idea that a geologist must necessarily fee one who amasses stones and comes home with a fresh burden from every excursion, is a popular but rather mischievous delusion. Field-geology does not mean and need not include the collecting of specimens. Consequently a formidable series of hammers and chisels, a capacious wallet with stores of wrapping paper and pill-boxes, are not absolutely and always required. Rock-specimens and fossils are best collected after the field-geologist has made some progress with his examination of a district. He can then begin to see what rocks really deserve to be illustrated by speci- mens, and in what strata the search for fossils may be most advantageously conducted. He may have to undertake CH. III.] ACCOUTREMENT FOR THE FIELD. 19 the collecting himself, or he may be able to employ a trained assistant, and direct him to the localities whence specimens are to be taken. But in the first instance, his own efforts must be directed to the investigation of the geological structure of the region. The specimens re- quired for his purpose in the early stages of his work do not involve much trouble. He can detach them and carry them off as he goes, while he leaves the full collection to be made afterwards. It is of paramount importance that the field-geologist should go to his work as lightly equipped as possible. His accoutrements should be sufficient for their purpose, and emin ently portable. The reader may j udge of th e por- tability which may be secured, when he learns that he may carry on his person, at the same moment, all the instru- ments necessary for a geological investigation, even in the detailed manner adopted in the Geological Survey of this country, and that yet, although a fully-equipped field- geologist, he need not betray his occupation by any visible implement. The want of such tokens of his craft often greatly perplexes rustic observers to whom his move- ments are a fruitful source of speculation. He may find himself, for instance, taken at different times and places for postman, doctor, farmer, cattle-dealer, travelling- showman, country-gentleman, gamekeeper, poacher, tem- perance-lecturer, gauger, clergyman, play-actor, and a generally suspicious character. One of my colleagues in the Geological Survey, who had just taken quarters in a village, was watched for some time by the police, under the belief that he had been concerned in a recent burglary. 1 1 On one occasion, in company with a Survey colleague, I reached a straggling village in the East of Fife, just after a travelling show C 2 2 o OUTLINES OF FIELD-GEOLOGY. [CHAP. 1. The Map. Unless the geological work to be done merely consists in visiting already known ground and making detailed notes, or collecting specimens there, it is of the utmost consequence to obtain as good a map of the region as can be had. Not merely does the observer find the advantage of the topographical guide over the ground, but, as I shall point out in a succeeding chapter, he cannot, in many cases, satisfactorily work out the geological relations of the rocks unless he possesses a map on which to place, in their proper geographical position, the notes he makes at each locality. Hence if he cannot procure a map, or if he is at work in a country which has not yet been topographically surveyed, he may find himself compelled to make a map for himself with as near an approach to accuracy as the means at his command will admit. 2. The Hammer. This is the chief instrument of the field-geologist. He ought at first to use it constantly, and seldom trust himself to name a rock until he has broken a fragment from it, and compared the fresh with the weathered surface. Most rocks yield so much to the action of the weather as to acquire a decomposed, crumbling crust, by which the true colour, texture, and composition of the rock itself may be entirely concealed. Two rocks, of which the outer crusts are similar, may had entered it. The villagers were still standing at their doors, dis- cussing the character of the new arrival, when we passed them. Of course we were naturally supposed to form a kind of rear-guard of the cavalcade ; but we had the satisfaction of hearing one old woman remark to her neighbour, as we brushed past them, " Na, noo, arena' thae twa decent-looking chields to be play-acting blackguards ?" in.] ACCOUTREMENT FOR THE FIELD. 21 differ greatly from each other in essential characters. Again, two rocks may assume a very different aspect externally, and yet may show an identity of composition on a freshly-fractured internal surface. The hammer, therefore, is required to detach this outer deceptive crust. If heavy enough to do this, it is sufficient for the purpose ; any additional weight is unnecessary and bur- densome. A hammer, of which the head weighs one pound or a few ounces more is quite massive enough for all the ordinary requirements of the field-geologist. When he proceeds to collect specimens he needs a hammer of two or three pounds, or even more, in weight, and a small, light chipping hammer, to trim the specimens and reduce them in bulk, without running a too frequent risk of shattering them to pieces. Hardly any two geologists agree as to the best shape of hammer ; much evidently depending upon the indi- vidual style in which each observer wields his tool. This (Fig. i) is the form which, after long experience we have found in the Geological Survey to be on the whole the best. A hammer formed after this pattern combines, as may be observed, the uses both of a hammer and a chisel. With the broad, heavy, or square end, we can break cff a fragment large enough to show the internal grain of a rock. With the thin, wedge-shaped, or chisel-like end, we can split open shales, sandstones, schists, and other fissile rocks. This cutting or splitting edge should be at a right angle to the axis of the shaft. If placed upright or in the same line with the shaft, much of its efficiency is lost, especially in wedging off plates of shale or other rocks. A hammer shaped as I recommend serves at times for OUTLINES OF FIELD-GEOLOGY. [CHAP. other than purely geological purposes. On steep grassy slopes, where the footing is precarious, and where there is no available hold for the hand, the wedge-like end of the hammer may be driven firmly into the turf, and the geologist may thereby let himself securely down or pull himself up. eological hammer, compass-case, and belt. The most generally convenient way of carrying the hammer is to have it in a leather sheath suspended from the waist-belt The hammer hangs at the left side under the coat, the inside of which is kept from being cut or soiled by the protecting outer flap of the sheath.. Some geologists prefer to carry the belt across the shoulders outside, and the hammer suspended at the back. Others in.] ACCOUTREMENT FOR THE FIELD. 23 provide themselves with strong canvas coat-pockets and carry the hammer there. 3. The Lens. Even the most sharp-sighted observer is the better of the aid supplied to him by a good mag- nifying-glass. For field-work a pocket lens with two powers is usually sufficient. One glass should have a large field for showing the general texture of a rock, its component grains or crystals, and the manner of their arrangements ; the other glass should be capable of making visible the fine striae on a crystal, and the minuter ornament on the surface of a fish-scale or other fossil organism. Applied to the weathered crust of a rock, the lens often enables the observer to detect indications of composition and texture, which the fresh fracture of the rock does not reveal. It sometimes suffices to decide whether a puzzling fine-grained rock should be referred to the igneous or the aqueous series, and consequently how that rock is to be coloured on the map. 4. The Compass. Any ordinary pocket compass will suffice for most of the requirements of the field-geologist. Should he need to take accurate bearings, however, a small portable azimuth compass will be found useful. This is the instrument employed in the Geological Sur- vey. It is carried in a leather case or pocket hung from the waist-belt, on the side of the body opposite to the hammer. (Fig. i.) The directions of the dip and strike of rocks, the trend of dislocations and dykes, the line of boundaries, escarpments, and other geological features are observed accurately, and noted on the spot at the time of observation, either on the map or in the note- book. A convenient instrument for light and rapid sur- veys, or reconnaissances, combines the compass and the 24 OUTLINES OF FIELD-GEOLOGY. [CHAP. next instrument I have to describe the clinometer. I shall refer to it again. 5. The Clinometer, or dip-measurer, is employed to find the angle at which strata are placed to the horizon an important observation in the investigation of the geological structure of a country, and one having frequently a special economic value, as, for instance, when it points out the depth to which a well or mine must be sunk. Various patterns have been proposed and used for this instru- ment. Formerly a spirit-level was commonly employed. But apart from the difficulty of rapid adjustment for the FIG. 2. Clinometer. requirements of the field, the spirit levels in the clino- meters were apt to get broken. A much more portable and serviceable form of clinometer may be made by the geologist himself. It consists of two thin leaves of wood, each two inches broad and six inches long, neatly hinged together, so as to open out and form a foot rule when required. On the inside of one of these leaves a small brass pendulum is so fixed that when it swings freely and hangs vertically, it forms an angle of 90, with the upper edge of the leaf to which it is attached. An arc, graduated to 90 on each side of the vertical, is drawn on the wood, in.] ACCOUTREMENT FOR THE FIELD. 25 or on paper or brass fastened to the wood, so that when the leaf is moved on either side, the exact number of degrees of inclination is shown by the pendulum on the graduated arc. The corresponding face of the opposite leaf is hollowed out just enough to let the two leaves fit closely, and keep the pendulum in its place when the instrument is not in use. This form of clinometer, made of boxwood and bound with brass, may be obtained of instrument makers. 1 It is light and strong, and its dura- bility may be understood from the fact that the instru- ment which I carry in the field, though it has been in constant use for more than twenty years, is as true and serviceable as ever. If at any time the geologist has occasion to lighten his equipment for some long mountain expedition, where every additional ounce of weight begins to tell by the end of the day, and where, therefore, for the sake of doing as much and holding out as long as possible, he should carry nothing that is not absolutely needful for his purpose, he may advantageously combine the pocket- compass and clinometer, in the one instrument to which I have already alluded. This convenient instrument is about the size of an ordinary gold watch. It consists of a thin, round, flat metal case, shaped like that of a watch, and covered either with a common watch-glass, or still better, with a flat disc of strong glass. Instead of figures for the hours and minutes, the white enamelled face of this geological watch is that of a common pocket- 1 Messrs. Troughton and Simms, London, Mr. J. Bryson, Edin- burgh, and Messrs. Spencer and Son, Dublin, supply this and the other instruments referred to in the text. 26 OUTLINES OF FIELD-GEOLOGY. [CHAP. compass. But the interval between each of the four cardinal points is divided into 90. On the central pivot, just underneath the needle, a small brass pendulum is placed, and a straight-edge of metal is soldered on one side of the outer rim of the watch-case in such a position that the instrument will stand on it if need be, and the pendulum will then point to zero. A simple piece of mechanism passing through the handle enables the observer to throw the needle off the pivot, or let it down, as he may require. 6. The Note-Book and Pencils. As it is impossible for a field-geologist to remember the details of all the obser- vations he makes on the ground, or to insert them on a map, he regards a good note-book as an essential part of his apparatus. From the nature of his work he has frequently occasion to make rough sections, or diagrams, and if possessed of the power of sketching, he has abundant opportunity of aiding the progress of his researches by jotting down the outlines of some cliff, mountain, or landscape. Hence his note-book should not be a mere pocket memorandum- book. A convenient size, uniting the uses of a common note-book and a sketch-book, is seven inches long by four-and-a-quarter inches broad. Let me remark in passing that perhaps no accomplishment will be found so useful by the field- geologist as a power of rapid and effective sketching from nature. If he has this power in any degree, he ought sedulously to cultivate it Even though he may never produce a picture, he can catch and store up in his note- book impressions and outlines which no mere descrip- tions could recall, and which may be of the highest value in his subsequent field-work. This is true of ordinary in.] ACCOUTREMENT FOR THE FIELD. 27 detailed surveys, and still more of rapid reconnaissances which may have their ultimate usefulness enormously increased if the observer can seize with his pencil and carry away, the forms of surface as well as the geological relations of the region through which his traverse lies. As every device which saves labour and time in the field, or which adds to the clearness of the work, is deserving of attention, I would refer here to the use of variously- coloured pencils for expressing at once, upon map or note-book, the different rock-masses which may occur in a district. Water-colours are of course ulti- mately employed for representing the geological .forma- tions on the finished map. But a few bits of coloured pencils carried in his pocket save the geologist much needless writing in the field. To a red dot or line he attaches a particular meaning, and he places it on his map without further explanation than the local pecu- liarities of the place may require. Such are the few prime instruments required in field- geology. We may add others from time to time, ac- cording to the nature of the work, which in each region will naturally suggest the changes that may be most advantageously made. A small bottle of weak hydro- chloric acid, carried in a protecting wooden box, or case, is sometimes of use in testing for carbonates, particularly in regions where rocks of different characters come to resemble each other on their weathered surfaces. When Sir William Logan was carrying on the survey of the Laurentian limestones of Canada, he received much help from what he called his " limestone spear." This was a sharp-pointed bit of iron fixed to the end of a pole or a walking-stick. He enlisted farmers and others 23 OUTLINES OF FIELD-GEOLOGY. [CHAP. in his operations, instructed them in the use of the spear, and obtained information which gave him a good general notion of the distribution of the limestone. The spear was thrust down through the soil until it struck the rock below. It was then pulled up, and the powder of stone adhering to the iron point was tested with acid. If, after trying a number of places all round, the observer uniformly obtained a brisk effervescence when the acid drop fell on the point of his spear, he inferred that the solid limestone existed below, and noted the fact on his map accordingly. When the Geological Survey was busy with the great Wealden area of the south-east of England, my col- leagues used what they nicknamed a " geological cheese- taster." It was indeed a kind of large cheese-taster, fixed to the end of a long stick. This implement was thrust down, and portions of the subsoil and of the clays or sands beneath were pulled up and examined. Similar devices must obviously suggest themselves according to the nature of the work in different districts and countries. In the course of his observations in the field the geologist will meet with rocks, as to the true nature of which he may not be able to satisfy himself at the time. He should in such cases detach a fresh chip from some less weathered part of the mass and examine it further at home. Detailed methods of investigation, which may be pursued with all the conveniences of a laboratory in town, are not possible to him in the country. But he may subject his specimens to analysis in two ways, and obtain valuable, and perhaps sufficient, information as to their characters. He can easily fit up for himself a in.] ACCOUTREMENT FOR THE FIELD. 29 small and portable blowpipe box, apparatus for preparing rocks, minerals, and fossils for examination, and a micro- scope with which to examine them. In Part II. of this little volume I shall enter into some details regarding these indoor employments of the field-geologist, and show how the apparatus may be put to practical use. 7. The Blowpipe Box should contain as much of the most useful apparatus as the space will admit, consistently with the whole box being easily packed into a port- manteau. The reader will find a list of the more essential articles in Chapter XVI. By means of the blowpipe it is often possible to determine the nature of a doubtful mineral or rock, and to ascertain the proportion of metal in an ore. A young geologist should take with him to the field only the most essential apparatus and re-agents ; he will gradually come to see by practice what additions he may best make to his equipment. Details on this subject will be found in Chapter XVI. 8. Rock-slicing Apparatus. Portable forms of slicing and polishing machines are now to be procured, though even the lightest of them add considerably to the traveller's baggage. The field-geologist may succeed, however, in preparing his slices by chipping thin splinters from the rock and reducing them in the manner described in Chapter XVIL, where instructions are given which it is hoped will enable him to supply himself with a micro- scopic slice of any rock he may encounter in the field. The labour involved in this process is well bestowed, for by means of the microscope, more than by any other method, he obtains an insight into the internal texture and arrangement of the rocks with which he is dealing. He sees what are the component minerals 30 OUTLINES OF FIELD-GEOLOGY. [CH. m. of a rock, and ho\v they are built up to form the mass in which they occur. He likewise can detect many of the changes which these minerals have undergone, and he thus obtains a clue into some of the metamorphic pro- cesses by which the rocks of the earth's crust have been altered. 9. Microscope. This instrument should be, like the rest, as portable as possible. For most geological pur- poses high powers are not required, consequently a small microscope is sufficient. Two powers i| and | inch focal length are extemely useful, and for the requirements of work in the field are quite adequate. An instrument with fairly good glasses of these powers, magnifying from 30 to about 300 diameters, according to the arrangement of object-glasses and eye-pieces, may be had of some London makers for ^5. It is sometimes of service, when working in a district where microscopic rock- sections are required, to carry a small collection of microscopic slices of selected or typical rocks or minerals, for purposes of comparison. A series of fifty or one hundred slices can be packed in a box a few inches square. 1 1 Typical series of this kind may be had from Fuess, of Berlin, or from Tennant or Gregory, London ; or Bryson, Edinburgh. CHAPTER IV. GEOLOGICAL MAPS. IN the foregoing list of a field-geologist's accoutrements, the map was put first. The propriety of assigning it this place of honour will be admitted when the real meaning and importance of a geological map are recog- nized, and when the observer can carry with him the map on which he himself has traced the geological boundary-lines. A published geological map is a valu- able guide when it can be had, but in the field-geologist's eyes its importance is but secondary compared with the map which contains perhaps the substance of his work for weeks or months together. The results obtained by the geologist in the field, from his investigation of the rocks, may be set down either in writing, or in maps and sections. No one can follow the practical pursuit of the science without being conscious how much his work gains in precision when he is com- pelled to put it down upon a map. Not only is his in- formation made more accurate, when he requires to trace the exact lines of geological boundary, but he is led to search in nooks and corners, of which he would not otherwise have suspected the existence, and thus he acquires a thoroughness of grasp attainable in no other 32 OUTLINES OF FIELD-GEOLOGY. [CHAP. way. The best field-geology is of that kind which careful and minute map-making requires. It is not, of course, imperative that an actual survey should be made by the geologist ; but he must proceed in such a way that his observations, if tabulated and placed upon a map, would make that map a good geological one. Since, then, the kind of work required in the prepara- tion of geological maps illustrates most completely the nature and methods of field-geology, I shall describe the construction of these maps as practised in this country. The reader will bear in mind that, though he may never draw a geological boundary line, nor take any part in a geological survey, he cannot attain excellence in . the practical pursuit of geology in the field, without going through the training which, if need be, would qualify him for becoming a professional geologist. How this should be the case will, I hope, become clear in the sequel. Let us first consider what a " geological map " is. The meaning now attached to this term differs very much from that with which it was associated not very many years ago. In the early days of geology, those who devoted themselves to this branch of science were mineralogists, rather than what we should now call geologists. They termed their subject " geognosy," meaning thereby to indicate their object to be the increase of their knowledge of the minerals and rocks of the earth. They constructed what they called " geognostical maps," on which the positions of marked varieties of minerals and rocks were shown, but without any attempt at accurate, or even sometimes approximate, boundary lines, and with no hint whatever of geological structure, which we now regard as one of the chief objects of geological maps. iv.] GEOLOGICAL MAPS. 33 A perfect geological map should represent ist. A full and accurate topography, with the form of the surface and heights in contour-lines, shading, or otherwise. The Ordnance Survey maps of Britain on the scale of six inches to a mile may be taken as an admirable example. 2nd. All geological deposits, from the most recent to the most ancient, which may occur in the district embraced by the map, with their boundary lines accurately traced, and the relation of their distribution to the external form of the ground clearly depicted. 3rd. The geological structure of the region, that is, the relation of the rocks to each other, their inclination downwards from the surface, their curvatures and dislocations ; in short, all particulars necessary to enable a geologist to apprehend the manner in which the rocks of the crust of the earth beneath the region in question have been built up. 4th. Information which may have special economic value, such as the nature and distribution of the soils, the position of available building materials, the direction, thickness, and extent of ores, coal-seams, or other useful minerals, the best sources of water supply, &c. To fulfil these various requirements the map must evidently be on not too small a scale. If the scale is small, the attempt to crowd a great deal of information into the map may result in confusion of detail, and most of the beauty and usefulness of the work may be lost. In such cases it is better, where practicable, to subdivide the labour, putting the older geological formations on one copy of the map, the superficial accumulations and soils on another, the industrial information on a third, and so on. But without attempting to express all the detail possible, we may construct a correct and serviceable u 34 OUTLINES OF FIELD-GEOLOGY. [CHAP. geological map of a district or country by generalising the information so as to give at a glance a broad and clear view of the distribution of the formations and the chief points of geological structure. The Geological Survey of Great Britain and Ireland is constructed chiefly upon field-maps (Ordnance Survey), on the scale of six inches to the British statute mile, or 10 5 65 - of nature, but some limited districts, where great detail is required, have been surveyed on the scale of twenty-five inches to the mile. The general geological map of the British Islands is published on the scale of one inch to the mile, or g-^^g-^ of nature. A convenient scale for a generalised map of a country is ten miles to an inch. Of course the smaller the scale the less detail is possible, and the more care must be taken to select those geological features which are of prime consequence. More important than the scale is the correctness of the topographical map which is to serve as the basis of the geological one. Unless the geography be accurately depicted, geological lines may be distorted, sometimes to an extent which seriously interferes with the value of the map. The importance of this point will be understood from two diagrams (Fig. 3), which represent the influence of correct and incorrect topography upon geological lines. It will be observed that the same district is represented in both drawings ; the streams and their tributaries are the same in both, but differ considerably in direction. A geologist trusting to the map A inserts the boundary lines between the formations i, 2, 3, 4, and 5, guiding himself by the points of intersection of the different streams. If now he were to trace these same lines on a map with the correct topography, as shown in B, he would find them to GEOLOGICAL MAPS. 35 present considerable differences from those on A, although crossing each stream at the same points on each map. In A his thick black line is a winding one, in B it is FIG. 3. Maps showing the effect of incorrect tepography in distorting geological lines. nearly straight. Should this boundary be a line of disloca- tion, the reader will see that by the one map he might be led to speculate upon a curved dislocation, in the other on a straight one. 36 OUTLINES OF FIELD-GEOLOGY. [CHAP. In this country we fortunately possess accurate Ord- nance maps on various scales, so that except in those few and remote districts of which the Ordnance Survey has not yet been completed, we have a good topographical basis, and may reach any degree of finish and complete- ness in geological map-making. It is useful, however, to be able to construct our own rough field-map, or to cor- rect a faulty one. For this purpose we avail ourselves of the ordinary methods of triangulation. We may measure, as accurately as practicable, a base-line along some level piece of ground, such as a river-meadow or a sea-shore. From each end of our measured line we take a bearing with an azimuth compass to some neighbouring object. The point of intersection of the lines of these two bear- ings gives the position of the object on the map. Hav- ing one or two triangles constructed in this way, we may continue triangulating the whole district and filling in the topography, so as in the end to produce a map which may not be quite accurate indeed, but which will probably serve our immediate purpose. In those parts of the world where no good maps yet exist, geological and topographical surveying are some- times conjoined. I may cite, as admirable illustrations of this union, the explorations of the river-courses of Canada by the late Sir William Logan, Director of the Canadian Geological Survey. He and his colleagues had to furnish themselves with canoes, attendant Indians, provisions, and hunting-gear, and push up unexplored rivers, winding through the dense forests of the province. They explored, mapped, geologised, and hunted, laying down lines of traverse which served as the base for future more detailed topography, and did vast service in opening up the iv.] GEOLOGICAL MAPS. 37 country. Still more elaborately topographical were the remarkable surveys lately carried out under Dr. Hayden, Geologist in charge of the Geological and Geo- graphical Survey of the Western Territories of the United States. Year by year valuable reports, drawings, and photographs by that able observer and his associates made known the geography, geology, natural history, botany, meteorology, ethnology, and antiquities of thou- sands of square miles of previously unexplored or but partially explored land. Having obtained or made as good a topographical map as may be attainable for his purpose, the observer is furnished with the first great requisite for geological surveying, and one of the most useful parts of the equip- ment of a field-geologist, whether he attempts any actual surveying or not. Next to accuracy, judgment, and patience, neatness of hand is desirable in the geologist who would work out the structure of a district and express that structure on a map. Even the largest scale map does not admit of very voluminous notes upon its area, and where the scale is small there may be hardly room for notes of any kind. Under these circumstances the observer will do well to practise with the finest point to his pencil, making the neatest and most legible writing. After a brief experi- ence he will find that he necessarily adopts a system of signs and contractions on his map, not only to save writing, but to prevent the map from being so over- crowded with notes as to become hopelessly confused. Every field-geologist insensibly invents contractions of his own. For the fundamental facts of geological struc- ture, however, it is eminently desirable that the same 3S OUTLINES OF FIELD-GEOLOGY. [CH. iv. signs and symbols should be used with the same meaning on all published geological maps. The subjoined dia- gram shows some of the signs used on the maps of the Geological Survey of Great Britain and Ireland. ~-| Horizontal strata. N X Inclined }f^ Undulating ,, Contorted Vertical \ i Anticlinal axis. Synclinal ,, Strike of cleavage. Direction of Gla- cial stria. J5 Lead. cf Iron - 9 Copper. FIG. 4. S:me useful signs in geological surveying. CHAPTER V. PRELIMINARY TRAVERSES OR RECONNAISSANCES CIRCUMSCRIBING CONDITIONS. HAVING now examined the various parts of the equip- ment of a field-geologist, let us proceed to notice what use he must make of them. At the outset I would remark that while the mere possession of good instru- ments cannot make a geologist, the want of them will not prevent a skilled geologist from doing good work. The training of years enables him to judge of rocks and angles, of dip and of trends of boundary so nearly ac- curately as to make him often independent of hammer, compass, and clinometer. In like manner long experi- ence quickens his eye to detect geological evidence where a less practised observer, though searching for information, would fail to find it. This difference of training tells greatly in all preliminary surveys, recon- naissances, or rapid traverses of a country. The geolo- gist who has already had many years of campaigning carries with him a faculty of grasping the salient features of geological structure, and directing his attention, on the march, to every available source of information which will help him to fill in the details of his section. If it 40 OUTLINES OF FIELD- GEOLOGY. [CHAP. were always practicable, the exploration of new regions, where the traveller is necessarily confined to his line of route, but where he has nevertheless to report on the geology of many thousands of square miles of territory, should be placed in the hands of men trained in geological surveying. That this arrangement would be of advantage will be, I think, admitted when we have entered a little more into the details of field-work. No questions are probably put so frequently to the field-geologist as these " How do you know what lies beneath the surface soil ? Do you dig or bore ? " When he replies that he neither digs nor bores, yet can usually infer with considerable confidence what must be the nature of the rock underneath, his statement is received with a look of bewilderment or a half-incredulous smile. But though the geologist does not usually dig or bore, he avails himself of every artificial opening he can hear of as offering any information with regard to the rocks beneath the surface. Every natural exposure of rock comes under his notice. If there is a coast-line, he makes a preliminary traverse of it, to ascertain the general nature of the rocks. He ascends one or more of the stream-courses for the same purpose. If there is any commanding hill in his district, he makes an early excursion to its top, that he may gain some general idea of the form of the ground and the probable dis- tribution of the geological formations, so far as may be indicated by the landscape. On such occasions he will find the very great advantage of being able to sketch in his note-book an outline of the landscape. By so doing, he fixes the features in his mind in their natural v.] PRELIMINARY TRAVERSES. 41 proportions; he has the original sketch to refer to and to recall impressions which cannot be preserved by written words ; and he has his attention drawn to those prominent features where probably he may meet with most interesting and profitable geological work. First jottings of this kind in a country never before visited, and of which the geological structure is still unknown to the observer, have for him a special interest and value. They retain for him the natural effect made on his eye and mind by the scenery, apart altogether from any explanation he may eventually be able to offer of the meaning of the features which he impartially sketches. With increasing experience of geological structure and practice in sketching it, these rapid drawings or notes gain in precision and fulness. At first, of course, the observer may expect to find innumerable difficulties in his traverses of a country. He may find it impossible to take in any general con- ception of the whole region ; everything seems lost perhaps in endless multiplicity of detail. But as he masters the detail, his power of grasping, at an early period in the examination of a district, the salient features of the geology, will steadily increase. In par- ticular, he will be gratified to discover that he can, with growing success, identify rocks and formations even from a distance by their outlines, colour, character of vegeta- tion, or other distinctive trait. His first surmises regard- ing the geological structure of the ground, made during his preliminary excursions, will thus come to be more and more sustained by his subsequent surveys. In later chapters it will be seen by what steps he may most profitably acquire this kind of experience. 42 OUTLINES OF FIELD-GEOLOGY. [CHAP. The nature and conduct of these preliminary exami- nations not only vary with the character of the geology and physical features of the country, they differ ac- cording to the extent to which the country is settled and populous, or trackless and unexplored ; according to the existence or absence of maps of the region to be ex- amined; according to climate and other obvious causes. Such peculiarities as these, which greatly affect the first general traverses of a country, are apt to influence all the subsequent more detailed work. As an illustration of the different conditions under which field-geology may be carried on, let me contrast the work of the Geological Survey of Great Britain and Ireland with that of the United States Geological and Geographical Survey of the Western Territories. In this long-settled and populous country we have abundant means of communication by road, railway, or steamboat between all or almost all districts. Villages and towns are scattered so numerously over the land that we seldom need be in any doubt as to obtaining good quarters and food. The penny-post and electric telegraph accompany us even into some of the most retired spots. Books, specimens, and instruments can be sent to us at a few days' notice. Of nearly every district in the British Islands we may procure detailed Ordnance maps, by which to make our way over the ground, and on which to place the results of our geological observations. Besides, the main features and much of the detail of British geology are already known, and have been ex- pressed with more or less precision upon published geological maps. We cannot, therefore, begin anywhere in this small country without some kind of general know- V.] PRELIMINARY TRAVERSES. 43 ledge about the formations and structure of the district we may propose to examine. There is still another element to be taken into account as determining the character and methods of field-geo- logy in Britain one which perhaps geologists themselves hardly sufficiently recognise the climate of the country. I do not believe that any one who has not daily occasion to be out for many hours in the open air, and whose avocations make him to some extent dependent upon the weather, can have any proper notion of how good the average weather of this country is, and how few thoroughly bad days there are in the year when he cannot secure even an hour or two of outdoor exercise. Our summers are seldom too hot to prevent the full use of a long July day. Our winters are so mild, and in many seasons bring so little snow, that if need be we may in most years carry on field-work up to the end of December, and renew it at the beginning of January. Such being the conditions under which field-geology may be prosecuted in Britain, it is evident that an observer may start for any district of the country alone and investigate its structure by himself. There is no occasion for combining a geological party, though that may be done if need be. In the organised field-work of the Geological Survey each officer has his own area assigned to him, and works out its geology himself, consulting, of course, from time to time his colleagues, who may be stationed in adjoining tracts, and arranging with them as to the joining-up of their various geological boundary lines. The extent of ground which can be examined and mapped in a year by one of the geologists of the 44 OUTLINES OF FIELD-GEOLOGY. [CHAP. Survey varies, not only with the capacity of the surveyor, but with the nature of the ground, whether level, easily traversed, and with comparatively few geological sections, or rough and high, laborious to climb or cross, and abounding in streams and crags, all of which must be examined and mapped. A man might complete the survey of half a county lying upon the chalk of the south-east of England before another could get over more than a part of a single parish in such intricate geological and rough mountainous ground as that round Snowdon, or that in many districts of Scotland. Let me place before the reader some statistics re- specting the rate of work in the Geological Survey of Scotland, where much of the ground is hilly and where the geological structure is often far from simple. The average annual area of ground geologically examined and surveyed by each officer in the field is not much below 100 square miles. This amount is performed by an average daily walk of from ten to fifteen miles, ex- clusive of Sundays, holidays, wet days, and the time spent indoors in reducing the field-work and preparing it for publication. The part of the year devoted to actual surveying may be set down as about 200 days, or it may be perhaps rather more than that. We see, then, that one of the members of the Scottish Geological Survey walks about 2,000 or 2,500 miles in the course of the year. Every square mile of his completed map represents, therefore, on the average, about twenty or twenty-five miles of actual walking. It will be readily believed, that with all the advantages for field-geology in Britain it should be possible here to construct the most elaborate geological maps. I would v.] PRELIMINARY TRAVERSES. 45 refer to some of the published sheets of the Geological Survey of the United Kingdom for an illustration of what can be, and has been, done in this respect. I do not suppose that any such detailed geological work has been elsewhere attempted. The large maps on the scale of six inches to the mile, with which the field-work is now chiefly conducted, admit of almost unlimited detail. Every important or interesting stratum may be put down and traced on these maps ; little dislocations of only a few feet in extent may be shown even when they are pretty closely crowded together ; no feature of geological value need be omitted for want of space to express it. As illustrations of intricate and detailed geological map- ping I may cite sheets 14, 15, 22, and 23 of the one- inch Geological Survey Map of Scotland, and the corre- sponding six-inch coal-field maps belonging to the same tract of the country. Now with field-geology and map-making as possible, and as actually accomplished, in Britain, let us contrast the conditions under which work of this kind must be carried on in an unexplored region like the Western Territories of the United States. The survey of vast tracts in those parts of the North American continent by Hayden, King, and Powell proves them to be among the most zealous, active, and efficient geologists who ever undertook the task of pioneering through a new country. But the utmost skill and experience cannot alter the natural features of a country and its climate. The American survey requires to be carried on in a very different manner from ours, and I cite it as an excellent example of how field-geology can be prosecuted in new and previously unmapped regions. 46 OUTLINES OF FIELD-GEOLOGY. L CHAP. As the topographical map of the country required to be made, Dr. Hayden's survey was at once geographical and geological. His staff contained more topographers than geologists. It required division into separate work- ing parties, to each of which a distinct tract of country was assigned. From the higher hill-tops triangulations were made and outline-sketches were taken, so that a general map was traced and filled in. In this work the geologists co-operated, indicating to their associates the salient geological features of each region, and inserting these upon sections or diagrams, which, for beauty and effectiveness, are among the most remarkable geological sketches which have yet been produced. Besides the scientific staff, however, provision had to be made for a foraging department : and sometimes, also, an escort has been needed, where the work lay in or near the territories of hostile Indians. As a sample of the equipment of Dr. Hayden's survey I may cite a few particulars from his Report for 1874. The staff in the field was divided into seven parties. Of the organisation of these, the first may be taken as a type. It consisted of one assistant geologist as director, two topographers, two meteorologists, one botanist and collector, one general assistant, two packers, cook, and hunter. It would seem that there was thus only one geologist in the party, though probably one or two of the other members were able to lend him some assistance. Starting on the aoth of July, the party continued the campaign till the 2;th of November. During that time it surveyed 4,300 square miles of new ground, which is probably an average of somewhere about forty square miles a day. This working party, therefore, though v.] PRELIMINARY TRAVERSES. 47 probably not much more than one geologist strong, accomplished in three days as great an area of work as one of my colleagues finds it possible to complete in a year. Such rapid surveying can of course be re- garded as furnishing merely a kind of rough prelimi- nary sketch of the geology of the territories, to serve as the basis for future detailed surveys. It may be taken as an example of broad generalised field-work on the one hand, while the Geological Survey of Britain stands at the opposite extreme, as a model of patient and elaborate detail. The student may usefully refer to other examples of such pioneering geological exploration in Western America. Of these, the " Exploration of the Fortieth Parallel," under Mr. Clarence King, and the " Geological and Geographical Survey of the Rocky Mountain Region," under Major Powell, well deserve perusal. The more recent monographs of Captain Button, of the United States Geological Survey, may also be profitably studied. CHAPTER VI. DETERMINATION OF ROCKS. WHETHER field-geology is to be carried on rapidly and in a generalised way, or slowly and in detail, the same methods must be followed. I have supposed the geolo- gist to have selected and reached his ground, and to have made a few preliminary traverses to gain some notion of the chief rocks and their arrangement. Let us follow his subsequent operations. The brooks, ravines, sea-coasts, hill-sides, valleys, and mountains, in short every natural section or artificial ex- posure of the rocks, will be carefully examined, and the observations made will be registered in note-book or map at the time. In the course of these rambles three points will have to be settled : first, the lithology and distribution of the rocks; second, their probable or actual geological horizon or date ; third, their position with regard to each other, that is, the geological structure of the district. The determination of the nature of the rocks is obviously the first question which must be dealt with. And here it must be remembered that the term rock is applied in geology indifferently to all kinds of naturally- ca. vi.] DETERMINATION OF ROCKS. 49 formed stones which occur in mass, even to peat, blown sand, and mud. Taking them in this wide sense, the geologist considers, with regard to those he encounters in the field, whether they are Fragmental, Derivative, or Stratified (Klastische Gesteine), and, if so, whether they are conglomerates, sandstones, shales, clays, limestones, ironstones, or other varieties of this great series ; whether, on the other hand, they are Crystalline or Igneous rocks, and if so, whether they should be classed as granite, syenite, diorite, basalt, gabbro, serpentine, or other species of this family ; or whether they are to be called Foliated or Metamorphic rocks, such as gneiss, mica-schist, or hornblende-slate. To be able to answer these questions, the observer must have trained his eye by the examination of good typical specimens of rocks. This is a kind of knowledge not to be obtained from books ; it can only be gathered from patient and intelligent handling of the rocks themselves. In the field the observer who has had this training in PETROGRAPHY, as the study of rocks is termed, can usually recognise the rocks he encounters. A pocket knife, lens, and acid-bottle will assist him if his eye does not readily detect the characters of the stone. But it will often happen that he requires to subject a rock to more careful examination at home, before he can decide as to its nature and name. It is absolutely necessary, however, that the field- geologist should have already familiarised his eye with certain important minerals which enter largely into the composition of rocks, so as to be able to identify and distinguish them, and thereby the rocks which they con- stitute. For this purpose he should procure a collection of these minerals, and subject them to careful examina- So OUTLINES OF FIELD-GEOLOGY. [CHAP. tion, so as to fix their characters in his mind ; while at the same time he will not omit to devote as much time as he can spare to the attentive study of any good mineralogical cabinet within his reach. The number of minerals which form essential constituents of widely- diffused rocks is comparatively small. Nor are those very numerous which occur abundantly as accessory or accidental ingredients. In Chapter XVI. the reader will find a list of those which it is desirable that he should know, with a reference to the part they play as constituents of rocks. But if the geologist means to devote himself to the study of the genesis of rocks, particularly those of igneous and metamorphic origin, he will find it needful to enter much more fully into the domain of MINERALOGY. Nor will he regret such an excursion ; for in studying the structure and growth of minerals he learns how rocks have been formed, and by what processes they have been altered since their formation. This is well brought out by the microscopical examination of crystals, as will be pointed out in a later chapter. Though practice alone can give the learner justifiable confidence among rocks in the field, some hints may be offered here for his guidance. He must learn to distin- guish between essential and accidental characters. Two rocks for instance may exactly resemble each other in colour, and even tn shade of colour, yet the one may be a derivative or sedimentary mass, the other an original or igneous one. Colour, therefore, can hardly be a very trustworthy index of the true nature of a rock. Again, a rock may at one place be so compact and tough as to be broken with difficulty, though at a short distance it may vi.] DETERMINATION OF ROCKS. 51 be as soft as loose gravel or sand. Wide variations in texture likewise occur ; a mass of rock will here present a coarsely crystalline or almost granitoid aspect, while there it may be so close-grained as to appear nearly homogeneous. In his field-work, therefore, the learner will discover by experience what are the essential characters in each case. Reserving more precise and detailed investigation for indoor-work (see Chap. XVI.), he will find that with the unaided eye and such instruments as can be carried in the field, he can take note of the following particulars of the rocks: i. Fresh fracture and weathered surface. 2. Structure and texture. 3. Hardness and streak. 4. Colour. 5. Smell. 6. Feel. 7. Behaviour in mass. I. Fresh Fracture and "Weathered Surface. All rocks yield more or less to the corroding action of the atmosphere. Some, like pure limestone, are merely dissolved by rain, and remain with a bare, clean, hard surface. But most of them show a more or less distinct FIG. 5. Weathered crust, showing concentric zones of oxidation. crust or outer crumbling skin, which is thicker or thinner, according to the resisting power of the rock on the one hand, and the vigour of the decomposing agents , on the other. In this outer weathered crust we may often E 2 52 OUTLINES OF FIELD-GEOLOGY. [CHAP. observe the composition of the rock better than on the fresh fracture. The very existence of such a crust depends upon unequal decay ; some one or more in- gredients of the rock disappear faster than the others, which may remain isolated and comparatively little altered in the crumbling debris of the decomposed con- stituent. For example, in many close-grained and crys- talline rocks, consisting of quartz and felspar, these two minerals are so intermingled and so resemble each other in colour and lustre, that at a first glance they might not be distinguished ; but on a weathered surface their clearly defined differences stand out very sharply ; the felspar has a dull earthy texture and white colour, while the quartz projects in hard glassy grains. A large number of rocks are characterised each by its own type of weathering. Thus, granite is apt to split along its joints and to assume, as it decays, the aspect of ruined walls and buttresses of Cyclopean masonry. Basalt rocks are prone to develop a spheroidal structure, each globular mass exfoliating into concentric onion-like coats (Fig. 6). Limestone projects in bare, smooth, bleached knobs, curiously fretted, channelled, and honeycombed ; the grass around is usually greener than elsewhere, and the ground is often perforated with swallow-holes, tunnels, and ramifying passages. The contrast between a weathered and a fresh piece of the same rock is often so extreme that the beginner would not willingly admit them to be from the same mass, unless he had himself detached them. Basalt, for instance, on a fresh unaltered fracture, is a com- pact or finely crystalline rock, heavy, and of an iron- black colour. But on a weathered cliff it may be seen DETERMINATION OF ROCKS. 53 of every hue from bright yellow to sombre brown, and in many places so soft as to be capable of being dug out with a spade. The beginner, therefore, should on no FIG. 6. Doferite (basalt) weathering spheroidally. North Queensferry. account omit to make himself acquainted both with the unaltered and the altered conditions of rocks. By degrees he will learn to recognise a rock through all its 54 OUTLINES OF FIELD-GEOLOGY. [CHAP. protean disguises of weathering, and distinguish it even at some distance. II. Structure and Texture. The nature of the component particles, and the manner in which they are arranged so as to build up the mass of the rock, con- stitute important characters. The geologist in the field has of course only very limited means of investigating these characters, so that when they become doubtful and obscure he may be compelled to defer the solution of his difficulties until he can find opportunity in-doors of sub- jecting the rocks to more detailed and careful scrutiny. ' But with the aid of his pocket-lens he can recognise three types of structure among rocks which may be termed respectively Crystalline, Compact, and Fragmental. i. CRYSTALLINE. In this type the rocks have a granular structure, and on inspection the apparent grains are found to be crystals, or crystalline particles, so intermingled, or felted together, as to give coherence to the stone. In the coarse-grained varieties, like many granites, the crystals of which can be distinctly seen at a distance of several yards, their true crystalline nature is at once apparent. We see that their grains are all crystalline, and that the lustre reflected from so many shining points on their surface comes from the cleavage planes of the component minerals. But as the texture becomes finer, as it does, for example, in the family of the basalt rocks, the unassisted eye may hardly be able to detect any crystalline facettes, even on a fresh fracture. The lens, however, will often show that such rocks really consist of very small crystals. But the fineness of grain may reach such a point as to escape detection even by that means, and then the observer must call the rock a compact vi.] DETERMINATION OF ROCKS. 55 one. It may still be quite crystalline, however, when examined under the microscope, in the manner described in a later chapter. We are at present concerned only with those external characters which can be recognised by the observer in the field. The crystalline particles are found to be built up on two different plans. In the great majority of rocks they are (i) amorphously aggregated, that is, they have crystallized together promiscuously without any definite arrangement, so that the rock presents much the same FIG. 7. A piece of grange. Crystalline structure. texture no matter in what direction it may be broken ; (ii) schistose or foliated, that is, disposed in more or less distinctly parallel folia or laminae. (ist) Amorphously aggregated. Rocks of this kind are a, Simple, or /3, Compound. (a) Simple. Composed essentially of one mineral, though now and then with accessory ingredients. The rocks of this sub-group are almost entirely of aqueous origin, that is, they have crystallized from solutions in water. Crystalline limestone, gypsum, and rock-salt may be taken as illustrative examples. A few silicates occur in this form, as hornblende-rock, but most of them incline to the foliated type. 56 OUTLINES OF FIELD-GEOLOGY. [CHAP. (/3) Compound. Composed of two or more minerals in an infinite variety of proportions. Most of the rocks which constitute this very important series are what are usually called Igneous ; that is, they have crystallized out of molten solutions like modern lava. They almost inva- riably consist of silicates of alumina, with magnesia, lime, potass, soda, and varying proportions of iron oxides, phosphate of lime, &c. The great majority of them are mainly composed of some felspar, or at least contain a large percentage of that mineral, with such silicates as hornblende, augite, olivine, biotite, and muscovite ; free silica in the form of blebs or crystals of quartz ; iron oxides, particularly magnetic and titaniferous ; apatite, &c. Hence they are commonly distinguishable from the simple rocks by their greater hardness, toughness, and weight. Granite, syenite, quartz-porphyry, basalt, diorite, are examples of compound rocks. Many varieties of texture occur among these rocks. The following are among the more important : Coarse- crystalline ; fine-crystalline ; (crypto-crystaWne, where the FIG. 8. Pece of lava, showing crystals and steam-holes. crystals are so minute as to appear only under the microscope, might be placed by the field-geologist among the compact series) ; porphyritic having large vi.] DETERMINATION OF ROCKS. 57 crystals, usually of felspar, scattered through a compact base ; cellular full of spherical cavities formed by the expansion of imprisoned steam during the outflow of the rock (Fig. 8) ; scoriaceous roughly and irregularly cel- lular, like the scoriae of a lava stream, or the " clinkers " from a foundry; amygdaloidal full of almond-shaped concretions of calcite, calcedony, zeolites, or other mine- rals ; these concretions having been deposited by infil- tration in steam-holes of the rock, so that when they weather out, the original cellular aspect of the mass is restored. (2nd) S c h i s t o s e. Rocks of this group are readily distinguishable by the peculiar arrangement of their component minerals into parallel layers or folia. These layers consist sometimes of one mineral, as in horn- blende-schist ; more usually they are composed of two or more minerals, as in mica-schist and gneiss. They may be observed to run into each other and to be as it were welded together. Yet they are distinctly crystal- line. In many cases they present a wrinkled or crumpled aspect, as if they had been puckered up by strong lateral pressure. ii. COMPACT. Without recognizable component crystals or particles, so far as can be made out in the field, but with a close, homogeneous texture. Three leading varieties may be noticed -ist, Glassy ; 2nd, Horny; 3rd, Fine-grained. (ist) Glass y resembling bottle-glass or pitch. This sub-group includes the natural glasses, as obsidian, and pitchstone. Some hydrocarbons, as asphalt and an- thracite, might be included here. (2nd) Horny having a feebly lustrous, translucent 58 OUTLINES OF FIELD-GEOLOGY. [CHAP. character, like flint. The chalk-flints and the cherts of older formations are good examples. (3rd) Fine-grained having a dull, exceedingly close granular texture, which may pass into the fine varieties of the crystalline amorphous rocks on the one hand, and of the fragmental rocks on the other. Many limestones and felsites show this texture. Hence, as it characterises rocks of very different geological structure and origin, it evidently must be used with caution as a means of iden- tifying them. Other characters should be looked for, and perhaps in the end appeal must be made to the microscope. iii. FRAGMENTAL (Clastic) composed of fragments of pre-existing rocks or minerals. As rocks of this type are mere mechanical mixtures, they present endless variety, both in composition and texture. In the vast FIG. 9. Piece of conglomerate, showing the characteristic rounded water-worn aspect of the component parts of many fragmental rocks. majority of cases they are of aqueous origin, that is, they have been laid down as sediment in water. Their com- ponent grains are therefore usually more or less rounded and water-worn (Fig. 9), even when consisting of crystal- vi.] DETERMINATION OF ROCKS. 59 line particles derived from older rocks. The coarse varieties, consisting of compacted gravel, are termed conglomerates when formed of rounded, breccias when formed of angular fragments. These coarse-grained rocks pass into grits and sandstones, where the ma- terials, usually more or less siliceous, have been ground down into sand. Argillaceous rocks are those composed of the finer or clayey sediment, sometimes arranged in laminae of deposit, as in shale, at other times with no fissility, as in fireclay and mud. An im- portant series of fragmental rocks has been formed by the consolidation of the loose dust and blocks ejected by a volcano. To these the general term Tuffs has been applied. (Fig. 10.) FIG. 10. Piece of volcanic tuff. With the fragmental rocks may be classed those which have been formed of the fragmentary remains of plants and animals. Ordinary crinoidal limestone is a characteristic example, consisting as it does of the con- gregated joints and plates of encrinites, with more or less perfect mollusca, corals, echini, fish-teeth, &c. Some of these organically derived rocks, however, pos- sess textures which would justify their being called 60 OUTLINES OF FIELD-GEOLOGY. [CHAP. compact rocks, as in the case of cannel coal. Others, again, have acquired, in large measure, a crystalline texture, as has happened so abundantly in the Mountain Limestone. The reader is referred to Chapters XVI. and XVII. for further information on this subject. ^^^^^^ FIG. ii. Piece of coal, composed of matted stems of Sigillaria. and Lepidodendron. Carmarthenshire. (De la Beche.) III. Hardness and Streak. Rocks differ much from each other in hardness ; even in the same mass of rock considerable diversities in this respect may be met with. Hardness is a character of secondary importance, though it may often be usefully employed to distinguish, among the compact rocks, siliceous from softer calcareous masses. Obviously it can only be properly applied to perfectly fresh surfaces, and is suited to homogeneous rather than to compound rocks. The scale of hardness employed in mineralogy may be used in testing rocks. DETERMINATION OF ROCKS. Talc. 2. Rock-salt. 3. CrJcite. 4. Fluor rpar. 5. Apatite. 6. Orthoclase. 7. Quartz. 8. Topsz. 9. Corundum (Emery) , 10. Diamond. A rock which can be easily scratched with the finger- nail, like many chloritic-schists, may be said to have one degree of hardness, or H i ; rocks possessing the hard- ness of rock-salt (2) can be less easily scratched with the finger-nail. The pocket-knife easily marks a limestone or crystal of calcite (3), which, on the other hand, re- sists the finger-nail; a little more pressure is required to mark a crystal of fluor-spar (4), and still more one of apatite (5). Rocks possessing the sixth degree of hard- ness can be scratched with the knife with difficulty, while when they possess gi eater hardness than about 6 degrees they resist the knife and even turn its edge, or take a streak of steel. Hence as rocks of this resisting power are almost always siliceous, the application of the knife furnishes a convenient means of discriminating them. Streak is the name given to the powder made when the knife (or file, or diamond) is drawn across the surface of a mineral or rock. Though sometimes useful in mineralogy, it is not often of much service among rocks. It may now and then be employed to distinguish compact dark bituminous clays or shales from varieties of coal, the former giving a dull brown or grey powder, and the latter a lustrous black streak. In the case of impure calcareous rocks, when little or no effervescence is visible in a drop of weak acid placed upon the clean surface, brisk disengagement of carbonic acid may often be produced by dropping the acid over 62 OUTLINES OF FIELD-GEOLOGY. [CHAP. the powder made by a scratch with the knife. Of course, individual minerals which occur either as original or accidental constituents of rocks may be tried for streak in the usual way required in mineralogical inquiry. Small specks of haematite may thus be detected by their characteristic cherry-red streak, while the iron- peroxide when hydrated will show its brown or yellow streak. IV. Colour. Great caution must be exercised in making use of this character in the discrimination of rocks. The same rock may, even within short distances, display the most extraordinary varieties of colour. But within certain limits the colour of a rock is an indication of the nature of some at least of its constituents. Iron is the great pigment to which the rocks owe their diversities of hue. It gives rise to numerous tints of yellow, brown, red, and green, as well as to blue and black. Some hints as to the causes of a few common varieties of colour may be of service. White. Limestones and clays are often quite white, and in this condition are almost always at their purest. Iron is generally absent, or present in but small quantity, in white rocks. The result of weathering is often to bleach rocks white, the air and rain removing the colouring materials, more especially the iron. The stones in a morass, or below peat turf, are commonly bleached as white as chalk on the outside the result of the reducing action of the organic matter on the iron oxides which are removed in solution as organic compounds or as carbonates. Black. Many carbonaceous rocks are black. Coals may be distinguished by their lightness, texture, and combustion. Clays or shales, rendered black by the yi.] DETERMINATION OF ROCKS. 63 vegetable matter they contain, may be recognised by their weight, streak, and their turning white but retaining their shape when strongly heated. But black heavy rocks abound in which there is no trace of carbon. These very generally contain a considerable amount of iron, either in the form of magnetite, ilmenite, or other related oxide, or in that of some black ferruginous mineral, such as hornblende. Such rocks are apt to weather with a brown or yellow crust, owing to the conversion of the iron into the hydrous peroxide. Brown. This colour characterises some rocks on their fresh fractures, as the variety of ironstone called black- band. A few crystalline rocks have a brown tint from the presence of minerals of that colour, such as varieties of mica and garnet. But it is more particularly on the decomposed surfaces and crusts of rocks that brown tints appear. The iron is there converted into the hydrous peroxide, limonite. Basalt rocks show this change in a most instructive manner. Earthy manganese also gives dark brown to black tints. Yellow. The colouring material of yellow rocks is almost always limonite. Yellow sandstones, beds of ochre, the weathered crusts of many limestones and of numerous ferruginous crystalline rocks furnish illus- trations. Sometimes a metallic or brassy yellow is communicated to parts of rocks by diffused iron-pyrites ; when this yellow is of the pale kind due to marcasite, it can only be seen on fresh fractures, as it disappears with the rapid decomposition of the mineral. Red. The prevailing hue of red rocks varies from a brownish-red to a bright brick-red, and is due to the presence of the peroxide of iron, haematite. Such rocks 64 OUTLINES OF FIELD-GEOLOGY. [CHAP. are often mottled with or pass into yellow and brown tints, where the iron they contain has been hydrated. These colours are most typically displayed among red sandstones and clays, of which an enormous mass occurs in the Old Red and New Red Sandstone, and in the Trias. Some rocks show a delicate flesh-red tint from the colour of their orthoclase felspar, as in pink granite. Iron is in this case also the pigment. Green. Many red sandstones are marked with circular spots of green, due to the reduction of the iron oxide. Protosilicate of iron is the prevalent green pigment of rocks ; carbonates of copper sometimes colour rocks of bright verdigris and emerald green tints. Many mag- nesian silicates are green, and impart green colours of various hues to the rocks of which they are constituents. Thus hornblende and augite give rise to dark bottle- green, and among the schistose rocks to paler apple- green and leek-green tints. The hydrous forms of these silicates, talc, chlorite, and serpentine, form characteristi- cally green rocks, the talc rocks shading off into white, and serpentine into black and dark red. Glauconite extensively diffused through certain sandstones gives them a characteristic green colour. Blue is not a frequent colour in rock masses. It is often spoken of as the colour of many limestones, which, however, are grey or bluish-grey. Beautiful belts of pale blue and white occur among the schistose rocks where the mineral kyanite abounds. Some clays and litho- marges are of a pale lavender-blue. Patches of a bright smalt blue, or of an indigo tint, may be met with among peat-mosses, where some animal organism has decayed and given rise to the formation of phosphate of iron. vi.] DETERMINATION OF ROCKS. 65 Grey may be said to be the prevailing colour among rocks, especially of the older geological periods. In simple rocks like limestones it is often produced by the intermingling of minute particles of clay, sand, or iron- oxide, or of amorphous carbonate of lime with the paler crystalline calcite of the comminuted organisms. Pure crystalline limestone is naturally snow-white, as in Carrara marble. In compound rocks the prevailing grey hues depend on the mixture of a white mineral, usually a felspar, with one or more dark minerals like magnetite, hornblende, or augite, the lightness or dark- ness of the hue depending upon the relative proportions of the constituents. Should the felspar be coloured by iron, a pinkish hue may be given to the grey ; or if the dark magnesian silicates have been altered into some of their hydrous representatives, the grey becomes more or less distinctly green. The old " greenstones " owe their distinctive hue to this source. V. Smell. Clay-rocks may be recognised by the peculiar earthy odour they give out when breathed upon. Crystalline felspar rocks when breathed upon often yield this smell. Some rocks, especially limestones containing animal matter or decomposing iron sulphides, yield a fetid or rotten-egg smell when freshly broken. VI. Feel. A few rocks are characterised by a peculiar feeling to the touch. This is chiefly shown by the hydrous magnesian silicates, talc, chlorite, ser- pentine, &c. (also by some micaceous schists), which have a greasy or soapy feel. In large tracts of country formed of chlorite-schist, margarodite-schist, or ser- pentine rock, the stones have everywhere this cha- racteristic. The term " trachyte " was originally applied F OUTLINES OF FIELD-GEOLOGY. [CHAP. vi.] DETERMINATION OF ROCKS. 67 to certain volcanic rocks distinguished by the harsh, prickly feeling experienced when the finger is passed over their surface. A rock like chalk is said to be meagre to the touch. VII. Behaviour in Mass. There are some re- markably characteristic aspects of rocks which cannot be judged of in hand-specimens, any more than the architecture of a building can be told from the nature of the stone employed in its construction. It is as parts of the architecture of the earth's crust that rocks present many of their most typical and individual features. These broader and larger characters show themselves in the outline of every hill and mountain. As illustrations we may take the two contrasted groups of the stratified fragmental and amorphous crystalline rocks. Even from a distance the difference between these rocks makes itself felt in the striking distinctions so often visible in the form of mountains. Thus in Fig. 12 it will be noticed that two prominent sets of lines can be traced all along the crests and declivities the horizontal lines of the bedding and the vertical lines of the joints. The rocks are cut into huge blocks in the process of denuda- tion, and these blocks are further channelled and chiselled along the dominant divisional lines. With this recti- linear style of architecture compare that of a mass of granite, one of the amorphous crystalline rocks. No parallel systems of lines here catch the eye. The crests are splintered, indeed, along the joints, and these divisional lines may be traced by a practised eye down many of the cliffs and steep declivities of granite, but they never show the definiteness, regularity, and alter- nation of prominent and retiring bands so typical of OUTLINES OF FIELD-GEOLOGY. [CHAP. vi.] DETERMINATION OF ROCKS. 69 stratified rocks. The general lines of the mountain are graceful curves rising more and more towards the summits till they often become vertical. The stratified rocks, then, are distinguished by their arrangement into beds, varying according to the nature of the substance, from the finest laminae up to large masses many yards in thickness. The amorphous crystalline rocks, on the other hand, are marked by the absence of all structure except their joints. The reader will find this subject further illustrated in succeeding chapters ; but he will learn more by a little practice in the field than can be easily communicated by books. CHAPTER VII. THE NATURE AND USE OF FOSSILS. IN probably the great majority of cases, it is the interest attaching to the remains of once living plants and ani- mals imbedded in the rocks which induces people to read geological books and to devote their time to the endeavour to gain some practical acquaintance with geology. But as a rule the practical work begins and ends with the gathering of the specimens. In the present chapter I wish to show that apart from their interest or beauty as specimens which can be arranged in a collection, the relics of former organisms are of the utmost value in geological inquiry, that in fact, so far as relates to the chronological succession of geological history, their importance is paramount. A " fossil " is literally anything dug up. The word, formerly applied indiscriminately to any mineral sub- stance taken out of the earth, whether possessing organ- ized structure or not, is now restricted to the remains or traces of plants and animals which have been imbedded by natural causes in any geological formation, whether ancient rock or modern superficial deposit. Thus under the designation "fossil," we must include the entire CH. vii.] THE NATURE AND USE OF FOSSILS. 71 carcases of mammoths and rhinoceroses which have been preserved for ages with their flesh and hair, frozen up in some of the muddy soils of Siberia ; the skeleton of a stag preserved in a peat-bog ; the scales and teeth of fishes scattered through a solid limestone ; the shells of mollusca, the calcareous framework of corals, the com- pressed leaves, fruits, and stems of plants ; in short, any and every part of an organism which has been imbedded in a geological formation, no matter what may be its condition of preservation, and whether or not it has been partially or wholly petrified. But not merely must we include every portion of the organism ; we may properly class also with fossils every substance or marking which has been connected in any way with the organism and bears witness as to its exist- ence and character. Thus, the resin of a tree, the trail or the castings of a worm, the droppings of animals, even the tools and weapons of man, may all become fossils and yield their evidence as to former conditions of life. As the circumstances under which fossils have been entombed have greatly varied, the observer must be pre- pared for the most extraordinary differences in the appear- ance of even the same species of fossil in different places and kinds of rock. In some rare examples the body of the animal has been so entirely and perfectly preserved that its flesh when first disinterred may actually be eaten, as was the case with the Siberian mammoth just referred to, which was so fresh as to be torn in pieces and devoured by beasts and birds of prey. As a rule, however, the soft parts of the organism are gone. Where there have been harder parts, such as an internal skeleton or an 72 OUTLINES OF FIELD-GEOLOGY. [CHAP. external covering, these may still remain nearly or quite in their original condition. As a rule, however, even the harder parts have undergone some change ; they have lost some portions of their original substance, more particularly the animal matter, and have had mineral material infiltrated instead. And the process of replace- ment has often continued until every particle of the original bone or shell or stem has been removed and has been replaced by carbonate of lime, silica, spathic iron, or some other mineral whereby the minute structure of the organism has been perfectly preserved. In other cases the whole of the animal or plant has disappeared, and has been replaced by a cast which retains the external form of the original, but is internally entirely structureless ; or the cast, if there ever was one, has been destroyed, and only an empty cavity remains to mark where the organism once lay. In the case of the mollusca we may have either a cast of the external or internal form of the shell. The observer will often be puzzled at first by such internal casts, as he will at once understand if he takes the two valves of an empty oyster-shell, places them in their original position, and, after making a small hole in one of them, pours in liquid plaster of Paris until the internal cavity is filled with it. When the plaster has set he can remove the valves, and he will have an in- ternal cast of the oyster. But had he seen the object before making the experiment, he would not have been likely to guess what it was. Again, at the outset he may experience some difficulty in identifying the same fossil when it occurs in different kinds of stone. For example, a plant which, when pre- served in shale or any argillaceous layer, may retain each vii.] THE NATURE AND USE OF FOSSILS. 73 leaflet, scar, and surface-marking, will perhaps appear in sandstone as a mere black streak of coaly substance. A fossil fish, which if found in a limestone nodule may have every scale and bone in place, each with its peculiar sculpture delicately shown, may, if met with in a conglo- merate, occur merely in scattered fragments, all so much rounded and worn as to be hardly recognisable. A little experience will guide the learner to those rocks which are likely to contain fossils. No general rule can be laid down ; for the kinds of rock which are barren of organic remains in some places, abound with them in others. Conglomerates, for example, are not usually rocks in which we should expect to meet with fossils ; nor as a rule do we find them there. Yet there are many richly fossiliferous conglomerates, such as those of the Silurian rocks of Penwhapple Glen in Ayrshire, and of the Upper Old Red Sandstone in several parts of Scotland. Argillaceous rocks are commonly better grounds for fossil-hunting than sandstones, and limestones are bet- ter than either. The shaly bands however which lie above a limestone are often more prolific than the limestone itself, as the fossils can be extracted entire from the soft, surrounding matrix. The inspection of a well-arranged series of fossils in a museum, all cleaned and neatly labelled, affords but small assistance in the practical work of finding the fossils in the rocks. The learner must betake himself to the localities from which he knows that fossils have already been obtained ; or if it is a district not yet explored for fossils, he must carefully note first of all the characters of the rocks. He will discover after some practice that it is not luck, but skill and good eyesight, which make 74 OUTLINES OF FIELD-GEOLOGY. [CHAP. the successful collector. Two observers may go over the same ground ; one of them diligently applies his hammer, breaks up innumerable blocks of limestone, finds not a single recognisable trace of a fossil, and pronouncing the rock to be unfossiliferous, passes on ; the other, perceiving the calcareous nature of the stone, and therefore its possibly fossil iferous character, puts his hammer in his belt, and betakes himself at once to the FIG. 14. Fossils standing in relief on a weatnered surface of limestone. weathered blocks. He knows, as every one soon does who attends to the subject, that in many cases a rock, which is really highly fossiliferous, may not appear to be so on a fresh fracture, where the whole texture of the stone may be uniformly crystalline. But when exposed to the slow corrosive influence of the weather, the difference between the molecular arrangement of the calcareous matter in the vii.] THE NATURE AND USE OF FOSSILS. 75 organic remains and in the surrounding matrix begins to appear. Shells, corals, and crinoids stand out in relief on the weathered stone, showing even some of their most delicate sculpturing, while the surrounding limestone has been slowly dissolved and removed. In this way a rock which may have been supposed to be unfossiliferous by one observer is shown by another of greater training to be full of fossils. Old walls and buildings, the refuse heaps of old quarries, the angular blocks strewn at the base of a cliff in short, all surfaces of rock which have been lying exposed for a long while to the gentle influ- ences of the air, ,rain, and frosts, may be made to yield their evidence as to the fossils in the rocks of a district. There are five important purposes to which the geo- logist can apply the fossils he may encounter among the rocks : first, to throw light upon revolutions in climate ; second, to restore former conditions of geography ; third, to detect former movements in the crust of the earth ; fourth, to afford horizons which serve to unravel geo- logical structure ; and fifth, to fix the relative geological date of rocks. i. Climate. Within certain limits, fossils may be em- ployed to show under what conditions of climate the geological formations of bygone ages were accumulated. We know, for example, that in the older tertiary periods in Europe the temperature must have been considerably higher than it is now, for in strata of that age we find among the fossil plants, forms of palm, custard-apple, laurel, fig, and numerous conifers ; together with remains of turtles, crocodiles, sea-snakes, tapir-like pachy- derms, and many mollusca belonging to genera now living in warmer seas than those of Western and North- 76 OUTLINES OF FIELD-GEOLOGY. [CHAP. western Europe. On the other hand, it can be shown that the general climate of Central and Northern Europe at a later time became quite arctic in character, for the remains of the reindeer and the musk-ox occur in superficial formations even far south in France ; bones of the arctic lemming, mammoth, woolly rhinoceros, and other northern mammalia, mark the cave-deposits and other surface accumulations in the South of England ; shells now extinct in our littoral waters, but still living in those of northern seas, abound in the clays which fringe the coasts of the West of Scotland. It must be borne in mind, however, that the argument from organic remains may be pushed too far. When we are dealing with species no longer living, we need an accumulation of evidence to warrant any deduction from them as to climate. Two species of the same genus may flourish under very different conditions of climate, as we may see from the fact that the Elephas primigenius or mammoth was a thick-furred northern form, though his modern representatives inhabit intertropical latitudes. Hence it is not by one species, but by the whole assem- blage of the plants and animals, or what is called the fossil flora and fauna, of a formation, that the climate in which the organisms lived must be judged. The further removed the fossils are from us in time, the more do they differ from living forms, and the less reliable are they as witnesses to climate. 2. Geographical Conditions. In most cases it is only from the character of the included organic remains that the conditions under which stratified deposits were laid down can be determined. By the evidence of fossils we may confidently identify former land-surfaces, lake- vii.] THE NATURE AND USE OF FOSSILS. 77 bottoms, and sea-bottoms, (i) Land -surfaces are re- vealed to us by layers of terrestrial vegetation resting upon what must once have been soil, and which still contains the roots of the plants that grew upon it. Stumps of trees in their position of growth, with, it may be, their fruits and leaves lying around, and even an occasional wing-case of a beetle, or the remains of a lizard or land-snail, furnish unimpeachable proof that the localities where they occur were once tree-covered tracts of ground. Hence the occurrence of such a FIG. 15. Ripple marks in sandstone, terrestrial layer in a group of strata proves that during their deposition a pause ensued, and their site became land. Traces of ancient shores, or at least of shallow water, are often preserved in ripple-marked surfaces of sandstones (Fig. 15) on which the trails or burrows of annelides may now and then be observed. If rain-prints (Fig. 1 6) are associated with rippled surfaces, they con- clusively prove the sediment to have accumulated on a shore. Further evidence of the occasional exposure 78 OUTLINES OF FIELD-GEOLOGY. [CHAP. of the deposits to air and sun is yielded by the desic- cation-cracks so commonly found among sand-stones (Fig. 17) ; while now and then, footprints of birds and different quadrupeds, impressed on the soft sand, com- plete the picture of quiet shore-conditions of deposit. (2) Lacustrine shells and cyprid cases point to former lakes. A layer of white marl full of decaying forms of lymnea, planorbis, &c. may often be found below the grassy surface of a flat meadow. Such a layer as certainly demonstrates that the meadow was once a lake, as if we FIG. 16. Rain prints on sandstone. had documentary evidence to prove that such had been the condition of the place within the last few generations. (3) Corals, and other zoophytes, mollusca of such genera as /ingula, cyprina, buccinum, and rissoa, fishes of, for example, the ray and shark tribes, point to marine condi- tions of life. The conclusion that any particular stratum must have been laid down on the sea-floor might not be warranted were it made to rest on merely a single fossil ; butwhen the whole character or fades of the fossilsof a rock is of a marine type, we may confidently infer that the rock vii.] THE NATURE AND USE OF FOSSILS. 79 was deposited on the bed of the sea. Certain forms of life have had a remarkable persistence in the ocean. Some of the living brachiopods, for example, are closely FIG. 17. Sun-cracked surface of red sandstone marked with footprints, Hildburghausen, Saxony. similar to those even of very early geological periods. These persistent forms, though they do not absolutely prove, yet give strong grounds for believing that, as they So OUTLINES OF FIELD-GEOLOGY. [CHAP. are all marine forms now, so they must have been marine from the beginning. And when they are found associa- ated with other forms belonging to recognisable marine types, the inference cannot be resisted. 3. Terrestrial Movements. The importance of organic remains as witnesses of movements of the earth's crust depends upon the limitation of the organisms when living to their own conditions of existence. A group of living sea-shells cannot be found in an inland lake, nor will a living terrestrial vegetation be dredged up from the sea-floor. If, therefore, marine forms of life must be FIG. 18. Limestone bared by lithodomous shells. taken as evidence of the presence of the sea and terres- trial forms as proofs of land, we are furnished thereby with an easily applicable and reliable test of change of level between sea and land, and a measure of its minimum amount. A natural terrace of sand and gravel, full of littoral shells, and extending along a coast-line at a height of 100 feet above the present sea-level, shows that sea and land must have shifted relatively to each other to the extent of at least 100 feet. It is generally agreed that in such changes of level it is the land, and not the level of the sea, which THE NATURE AND USE OF FOSSILS. Sr moves up or down. We say, therefore, that the ter- race marks an upheaval of the coast to the extent of 100 feet. Barnacles adhering to rocks, and living shells (Fig. 1 8) which have perforated them, furnish equally satis- factory proofs of a rise of the land. On the other hand, a submergence may be demonstrated to have taken place when a terrestrial surface, with its tree-stumps in situ, ohl soil and sylvan leaf-mould, is found below high- water mark. The trees must have grown above the FIG. 19. Section of a buried land-surface (De la Beche). ee. rocks underneath ; (id, old vegetable soil ; aa, stumps of trees still erect in position of growth ; b. prostrate tree-trunk ; cc. horns of oxen and deer. The whole buried under silt and modern soil, /. limit of ordinary tidal action, so that the amount of depression must always more or less exceed the ver- tical distance between the line of the submerged trees and the upper edge of the beach. Among the geological formations which form the vi- sible part of the earth's crust, it is sometimes possible to obtain instructive sections wherein successive terrestrial movements and conditions of physical geography are well illustrated. A good example occurs in Joppa Quarry, G 82 OUTLINES OF FIELD-GEOLOGY. [CHAP. near Edinburgh. It will be seen from the accompanying section (Fig. 20) that five seams of coal occur, each repre- senting a terrestrial surface, or at least an aquatic floor whereon grew a vegetation with its roots in the water and its branches in the air. There must have been a progressive subsidence until the first formed coal- seam had been buried under many feet of sand and mud FIG. 20. Section of inclined Carboniferous rocks, Joppa Quarry, near Edinburgh. a, coal seams : b, limestone with marine organisms. The dotted bands are sandstones, the shaded are shales and clays. which inclosed also the remains of other similar terres- trial surfaces. At last, by a more prolonged submerg- ence and the clearing of the water, marine forms of life, zoophytes, encrinites, and molluscs, made their way into the area, and flourished so long as to form a bed of lime- stone about three feet thick. Subsequently the sediment returned, and as the water was filled up, new coal-growths sprang up as before. The observer will find it sometimes possible, by means of fossil evidence, to prove that strata, apparently in their natural order, have really been turned upside down, so that what seems the top of each stratum is really the bottom. This could be shown if we found in one of these strata, a row of fossils in their positions of growth, but with their lower ends uppermost. Suppose, for ex- ample, that one stratum contained many erect stems of trees, and that in every case the roots of these stems vii.] THE NATURE AND USE OF FOSSILS. 83 branched out freely at the upper end into an overlying stratum, evidently an old soil. We could not, in such a case, come to any other conclusion than that the whole of the rocks had been overturned. Again, instead of a series of land-plants, imagine a number of bunches of coral, with their roots still in the position of growth, but turned up to the sky. We could only explain that position by admitting that the rocks must have been inverted. 4. Geological Horizons. Fossils have often a high im- portance in affording to the geologist a clue in his endeavour to unravel the geological structure of a re- gion. He may discover, for example, that some parti- cular stratum, marked by the occurrence in it of certain fossils, can be recognised and traced over a considerable breadth of ground. He follows this stratum, using it as a datum-line from which to work out the arrangement of the series both above and below it. This use of fossils will be more evident when we come to deal in a later chapter with the tracing of geological boundaries, and the working out of geological structure. 5. Geological Chronology. To fix the relative geolo- gical position of rocks, and thus to establish a suc- cession or chronology, is doubtless the most import- ant service which fossils render to geology. Mere resemblances or differences in mineral character are seldom good for great distances. We cannot always be sure, simply on the ground of general petrographical resemblance, that a group of strata on one side of a country is identical with a similar set on the opposite side. If they closely resembled each other in that re- spect, but contained totally distinct fossils, we should , 2 84 OUTLINES OF FIELD-GEOLOGY. [CHAP. generally conclude, in spite of their outward similarity, that they could not be identified with each other, but must belong to different periods of geological time. Each great stratified formation of the earth's crust is distinguished by its own characteristic fossils. A method is thus obtainable of recognising the relative geological date of fossiliferous rocks. To determine and name fossils is the task of the palaeontologist. As a rule the field-geologist can do this only to a limited extent, though the greater his power in this res pect the more valuable his services in the field. Part of his train- ing, however, should consist in the study of as good a series of typical fossils as he can consult. He ought to familiarise his eye with the leading genera and more characteristic species of each geological system and for- mation. Knowledge of this kind, so portable when carried in the head ready for use, so bulky and difficult to transport and use when contained in many learned volumes, enables him to decide for himself as to the geological horizon of the formations. Should he be in doubt about the determination of his fossils, he must submit them to an expert in the subject. For many purposes of field-geology it is not absolutely necessary, though it may be very desirable, that we should know the names and the zoological or botanical grade of the fossils. What we need to know in the field is that cer- tain organic remains, whatever be their nature or names, occur in particular beds of rock. We should be able to recognise them and use them as indices to mark out the strata, and thus to fix our geological horizon. William Smith, by whom this stratigraphical use of fossils was origi- nally taught, knew little of the nomenclature or natural vii.] THE NATURE AND USE OF FOSSILS. 85 history of the fossils he dealt with. But he learnt to recog- nise them, and to judge accurately of their position in the geological series, and he made as admirable use of them in tracing the outlines of the development of the Second- ary rocks across England as if he had been able to name and describe each species. Geology has made vast strides since his time. Though the field-geologist may use the fos- sils without any scientific knowledge of them, the sooner he obtains that knowledge the better for his work. The broad outlines of William Smith's days have to be filled in by more minute and exhaustive work now. In fine, the field-geologist will find in all quarters of the world that an acquaintance with fossils can be turned to profitable account. It enables him at the outset to fix more or less definitely the relative age of the rocks among which he is engaged and thus affords means of comparison with the corresponding rocks of other countries. Where his labours are of no ambitious kind, but where he works for the quiet pleasure and open-air life of the pursuit, the study of organic remains affords him an endless fund for delightful meditation. They show him at one place evidence of an old sea-bottom, in the strata where marine remains are crowded together. At another locality they bring before him, in fresh-water shells and other forms, the traces of long-vanished lakes and rivers. At a third spot they reveal, by successive layers of compressed vege- tation and hardened loam, the gradual depression and submergence of old forest-covered lands. In such cases they suggest the lines along which his further search should be prosecuted for additional corroborative testi- mony as to the ancient aspects of the district in which 86 OUTLINES OF FIELD-GEOLOGY. [CH. vn. he is at work. The land-plants, for example, lead him to look for fresh-water forms of life, for sun-cracked and rain-pitted surfaces of rock; while the occurrence of marine forms of life prompts to search for other proofs of the ancient encroachments of the sea. CHAPTER VIII. THE TRACING OF GEOLOGICAL BOUNDARY- LINES. WHETHER or not the observer sets about the construction of a map, he can form but a limited notion of the geology of a country if he confines his attention merely to a few quarries or lines of natural section. Having learned in such openings what is the nature and order of succession of the rocks, he ought to try to follow them out, from where they are clearly seen into other parts of the country, and in so doing, endeavour to note as he goes any variation in character which they may present, and every feature which serves to indicate what must be the disposition of the rocks below. A very short experience of geological work in the field suffices to show the observer that over wide spaces he cannot actually see what rock lies beneath him. He may get an admirable section laid bare in some ravine or brook, or by the shore of the sea : but beyond the limits of this section the ground may be deeply buried under vegetation, soil, sand, gravel, clay, or other superficial formation, and no other section may occur for an interval of, it may b2, several miles. Yet he must form some 88 OUTLINES OF FIELD-GEOLOGY. [CHAP. conclusion as to the nature of the rocks between these places. In cases of this kind information may often be obtained from an examination of the soil. What we call vegetable soil is merely the upper stratum of decayed rock mixed with vegetable and animal remains. (Fig. 21.) It com- monly betrays its origin by the still undecomposed frag- ments of stone mixed through its mass. In one tract, for instance, we may find it full of pieces of sandstone, to FIG 2i. Section to show the superficial covering of soil (i), ubsoil (2) derived from the disintegration of the underlying rock (3). the exclusion perhaps of every other kind of rock. If the land has been under cultivation, the sandstone may be in large pieces, where it has been turned up by the plough. We should there infer with some confidence that sandstone lay underneath in situ. If again the soil were a stiff red loam or clay, with few or no stones, it would indicate the existence of some red marl or clay imme- diately underneath. A sandy soil full of well-rounded, water-worn stones, would show the presence of some gravelly deposit below. A calcareous soil full of blocks viii.] GEOLOGICAL BOUNDARY-LINES. 89 of flint would probably indicate the existence of chalk. A stiff argillaceous soil, abounding in smoothed stones, many of them well-striated, would prove that a boulder- clay or till lay below. A profusion of fragments of some peculiar rock, a basalt, for example, or a diorite, or a porphyrite, extending in a definite band across a field or hill-side, would probably show us that a rock of that character existed, in situ, somewhere in the immediate neighbourhood of the fragments. We require, of course, in all these cases, to go carefully over the ground, and draw our conclusion only after we have exhausted all the evidence procurable. But it may be remarked that, except on freshly-ploughed land, the soil is not bare and exposed to our scrutiny ; that, on the contrary, it is commonly just as much con- cealed by its coating of vegetation as the hard rocks are by their covering of soil. Even under the most unfavourable circumstances, however, the geologist may often be able to learn not a little of the information he needs. Where the ground has a slope he will probably have no great trouble in finding some little rut or trench which has been cut, or at least deepened, by rain, ar-d where he will obtain access to the underlying soil, or even, it may be, to the subsoil and the still undecomposed rock below it. Where, on the other hand, the ground is too flat to hope for assistance from rain-action, he will look for traces of burrowing animals, by which the soil may have been thrown up to the surface. In Britain the common earth-worm, the mole, and the rabbit, are excellent coadjutors in his work. The fine castings of the earth- worm give him at least the colour and general constitution of the soil, whether sandy or clayey. The heaps of the 90 OUTLINES OF FIELD-GEOLOGY. [CHAP. mole include the smaller stones in the soil, and permit an inference to be drawn as to the probable nature of the materials from the decomposition of which the soil has been formed. The extensive excavations of the rabbit lay bare not only the constitution of the soil, but often also the angular debris which rests immediately upon the solid rock. From vegetation, also, the field-geologist learns to draw many a shrewd inference as to the character of the soil and rock below. A spring, or line of springs, indi- cative of some geological boundary line such as the FIG, 22. Section of a valley showing the outcrop of a junction of sandstone and shale marked by a line of springs, s, s. junction of a harder or softer stratum (Fig. 22), or a line of fracture (Fig. 34), will reveal itself by marshy ground or by a brighter green along a hill slope. The course of a limestone band or a basalt dyke may be followed, by the peculiar verdure of its vegetable covering, across a moorland where little or no solid rock may be seen. A ridge of serpentine stands up bare and rough, affording at best but an unkindly soil for plant-growth. Trees, too, change with the varying character of the rocks on which they grow. Each country presents its own illustrations of these relations, which must be gradually learnt and made to give their assistance to the observer's progress. In judging of the probable character of the rocks GEOLOGICAL BOUNDARY-LINES. 91 underneath from the nature of the overlying soil, the geologist will, of course, be guided by the local circum- stances in every case. For example, if the surface of the ground should present many rounded pebbles and boulders, he will not at once conclude that these frag- ments have been derived from the rock in situ below. Their rounded forms will rather raise a suspicion 'that they have been transported, and should many of them plainly show the characteristic smoothed surface of water- worn stones, they will be set down as derived immediately from some adjacent bed of gravel or conglomerate. The mere fact of a great variety of rounded rock-fragments occurring over the surface at any locality, suggests a mass of transported material, rather than the decomposition of the solid rocks underneath. On the other hand, the occurrence of abundant angular fragments of rock on the surface, at once arrests attention, as indicative of the vicinity of that rock in situ. The observer traverses the ground in all directions in search of any projecting knob of the actual rock itself. Failing to find it, he notes the position of these angular chips, and tries whether they can be traced further, so as to indicate by their distribution at the surface the probable trend of the solid rock underneath. In ascending a hill- side so covered with trains of detritus or vegetation that no rock can be seen in place, the geologist may learn much regarding the concealed rocks by examining the debris. He knows that the fragments of stone have all rolled down, and not up. When, therefore, in his ascent, he observes that the angular chips of some particular rock, abundant enough below, no longer appear, he sur- mises that he must have crossed the limits of the solid 92 OUTLINES OF FIELD-GEOLOGY. [CHAP. rock which furnished the fragments. If in the course of subsequent examination he discovers that those fragments disappear about the same line all along the hill, he may regard his first surmise as probably correct, and draw a boundary line accordingly, even although he may never have seen the actual rock itself in situ. Again, in the ascent of streams similar close observa- tion and sagacious inference will often go far to supply the place of actual sections of rock. The use of the evidence in these cases, however, requires still more caution than on the bare hill-side, because the tendency of running water is to round the rock fragments exposed to it, and hence in the channel of a brook or river, it may not be always possible to distinguish between the pebbles which came as angular fragments from neighbouring solid rocks, and have been rounded by the attrition of the brook or river itself, and those which, derived from some old gravel, were already rounded and water-worn before they tumbled into the channel. A great abundance of fragments of one particular variety of rock, however, would suggest that they had not been washed out of some gravel bed, but had been derived from the waste of a solid rock lying somewhat further up in the drainage basin of the stream. In such a case, moreover, the proportion of these fragments in the channel would probably be found to increase as the stream was traced upwards. Perhaps they might be observed, too, to become larger in size and less water-worn the further they were followed up the stream. If they should suddenly cease, the observer should at once note the fact, as possibly indicating that the rock did not occur higher up, but had its upper limit somewhere near the viii.] GEOLOGICAL BOUNDARY-LINES. 93 point where the fragments in the stream disappeared. While these particular rock-chips ceased, others of some different rock might be found to increase in number, and another zone of rock might be shown and traced in a similar way. In nothing is the highest type of a field-geologist better displayed than in the exhaustiveness and sagacity with which, in the absence of all other evidence, these various little indications of the geology of a district are sought for, found, and marshalled in their proper places, so as to bear witness to the distribution and probable structure of the rocks. Such an observer would be able in many cases to trace lines, with a near approach to accuracy, over ground which a less skilled student would pronounce to be a blank. It must often happen, however, that the ground is so obscured by superficial accumulations, such as vegetation, soil, gravel, and clay, that no indication whatever can for considerable intervals be found as to the nature of the solid rocks underneath. Under these circumstances the geologist, when no boring or mining operations are at his service, must do the best he can, by examining all the surrounding ground, to determine what lies below the concealed area. And in the great majority of cases he can form a tolerably correct surmise as to the general nature and disposition of the rocks. To do this requires some knowledge of geological structure, which we shall consider in the following chapter. CHAPTER IX. THE UNRAVELLING OF GEOLOGICAL STRUCTURE. DIP, STRIKE, OUTCROP. IF we could only recognise the rocks, where actually seen, but form no satisfactory conclusion regarding their distri- bution under a concealing mantle of vegetation or super- ficial detritus ; if we could tell the arrangement and measure the thickness of strata only at the surface, but offer no opinion as to the prolongation of these strata underground, we should never know much about the crust of the earth, and certainly could do comparatively little to advance geological inquiry. Fortunately it is not only possible but comparatively easy to pronounce upon the subterranean arrangement of rocks from in- dications obtainable at the surface. We seldom need to bore or dig. It is usually enough ff we can avail our- selves of the surface evidence, and gain from it inform- ation respecting the probable arrangement of the rocks below, or in other words, the geological structure of the ground. How this is done, let us now proceed to consider. Horizontal. Strata. Outcrop. In a region where the rocks are all horizontal, only the uppermost stratum may CH. ix.] GEOLOGICAL STRUCTURE. 95 be seen, in which case an example of extreme simplicity of structure would result. There would then be no out- crop or exposed edge of any stratum to be traced, unless the surface of the ground should be so uneven as to ex- pose the edges of lower strata. Except in tracts of low alluvium, however, horizontal strata have usually been more or less trenched by valleys and ravines, so that sections are laid bare of the underlying beds, while the surface of the country seldom rigidly corresponds with the surface of a stratum, but has been worn across it, so as here and there to leave " outliers " or outstanding por- tions of this upper stratum, and to lay bare the strata below. Where this has taken place in bare hilly land, with abundance of exposures of the rocks, although the geological structure is still of the simplest possible kind, considerable practice and skill may be needed to follow the exposed edges or outcrops of the strata, and to de- lineate them accurately, and at the same time artistically, upon a map. The accompanying drawings (Fig. 23) may serve to illustrate how very tortuous the outcrops of perfectly horizontal beds may be, should the ground be much varied in outline, and especially by the occur- rence of wide and deep valleys. In the uppermost map (A) a representation is given of horizontal rocks deeply trenched by valleys and ravines. In the lower map (B) the inequalities of the ground are much less, yet even in such a gently undulating district the outcrops of hori- zontal strata may evidently run in remarkably sinuous lines. I have used the word artistic with reference to the tracing of geological boundary-lines, and have done so advisedly. Where the rocks are all visible, the observer OUTLINES OF FIELD-GEOLOGY. [CHAP. has only to follow nature, and the more faithfully he does so, the more graceful will his lines probably be. The curves produced by denudation, though often complex, FIG. 23. Sinuous outcrops of horizontal strata depending on inequalities of surface. are never awkward and inharmonious. Where the rocks are not seen, and where therefore the position of the boundary-lines must be inferred, the surveyor will follow ix.] GEOLOGICAL STRUCTURE. 97 the analogies of his district and run his boundaries with the same kind of flowing lines which he sees them to possess where they can be actually examined. Two men may map the same piece of ground quite correctly as regards its general structure, but the map of the one will show by the complexity of its lines and the fidelity with which they follow the varieties of the surface configura- tion, how faithfully and skilfully the work has been done ; while the map of the other will indicate that its author, though marking correctly the general structure, has failed to recognise, or, at least, to express the relations of that structure to external form. The former map will in most cases be a far more artistic as well as accurate produc- tion than the latter. Not only in such simple work as the tracing of horizontal strata, but in all the details of geological map-making, the artistic eye and hand have scope to show their presence : to the great advantage of the maps to which they are applied. Inclined Strata. Dip. Instead of lying quite flat, however, stratified rocks are usually inclined to the horizon. This inclination, called their dip, is measured as to its direction by the compass, as to its angle by the FIG. 24. True dip concealed by superficial disturbance of the strata. clinometer. In determining these points it is always desirable to see more than a mere projecting edge of rock, for sometimes what seems to be the dip in such a case is deceptive. In Fig. 24, for instance, the rocks are H OUTLINES OF FIELD-GEOLOGY. [CHAP. really inclined at a high angle towards the left hand. Yet if seen merely at the surface where they have been bent back (by the slide of rubbish down-hill, or by a grinding mass of ice or other superficial agent) they might be supposed to be dipping from left to right. To be sure of the true angle and direction, we must not be content FIG. 25. Inclined strata appearing horizontal when exposed at a right angle to the dip. with one small face of rock, but should go round a sec- tion until we determine the point satisfactorily. A face of rock, for instance, seen from one side, as in Fig. 25, may appear to be made of horizontal strata, which from another point of view are found to be considerably in- clined. The direction of dip will always be at a right angle to the line along which the edges of the inclined beds appear horizontal. Failing, therefore, to find any rx.] GEOLOGICAL STRUCTURE. 99 actual section along the true line of dip, we should so place ourselves as to have the exposed edges of the strata running in horizontal bars in front of us. We may then take the direction of dip with the compass, and determine from the mean of a number of observations taken with the clinometer on projecting ledges what must be the general average angle of dip. The best measure- ments of the angle of dip are made when we can place ourselves some little distance in front of a face of rock which has been cut in the true direction of dip. We can then place the clinometer in front of our eye, and make its edge coincide with the line of a particular stratum many yards in extent. Thus in one single observation we obviate the risks of error where only small ledges of the inclined beds can be used. Where the true dip cannot be directly measured we may, by measuring the apparent dip of two faces of rock inclined at a consider- able angle to each other, obtain the true dip by calcu- lation. 1 Having ascertained these particulars, we insert the in- formation in our note-book or map. The use of a map for the registering of observations on geological structure requires an amount of precision which might not be thought needful for the pages of a note-book, and secures in consequence the most careful and exhaustive kind of field-work. I shall, therefore, suppose in what I have to say on this part of my subject, that we are required not 1 Rules are given for measuring or calculating the dip. (See Green> Geology, p. 341.) For almost all practical purposes, how- ever, a good field-geologist can get his angle with the clinometer in the field by selecting, as he learns how to do, his points of observation. ioo OUTLINES OF FIELD-GEOLOGY. [CHAP. only to make observations on geological structure, but to formulate them on paper, and to construct the geological map of a region. The usual sign used on geological maps to express the dip of strata is an arrow pointing in the direction of in- clination (the direction being fou nd on paper by help of an ordinary protractor), with the number of degrees of angle shown in figures at the side of it. We place, therefore, an arrow at each point on the map where we ascertain the dip of strata. A glance at the map (Figs. 26 & 27) will show how this is done. Each arrow marks the site of the observation, and with its accom- panying figures records the result. Where possible we enter beside the arrow some symbols, or contracted writing, to describe the nature of the rock, or any other particulars which it seems desirable to record. Further detail, where required, finds its place in the note-book. Selection of Horizons Mapping of Outcrop. As it is impossible on any ordinary map to represent every bed of rock, the geologist must decide what beds should be selected to be traced out. This cannot always be done until considerable progress has been made with the work. The selection must depend not merely upon the geolo- gical or industrial importance of the beds, but also, and not less frequently, upon the extent to which they are exposed and capable of being followed across the district. A particular stratum of no special interest in itself may come to have a high importance as a geological horizon or platform if it is easily recognisable, and from its thick- ness, hardness, or other peculiarity, stands out so promi- nently that it can be satisfactorily traced from point to point for a long distance. Such stratigraphically serviceable ix.] GEOLOGICAL STRUCTURE. 101 bands may be found in most districts of stratified rocks Great assistance in the tracing of horizons is likewise afforded by organic remains, as has been already pointed out. A particular stratum, even when thin and other- wise of no apparent importance, may acquire a high value if it is charged with fossils, and can be recognised over a wide area. The outcrop may be marked at any particular locality by a short line beside the dip-arrow, or if the outcrop be a broad one, by two lines, one marking the base, the other the top of the band. The space between two such lines in other words, the breadth of the outcrop is determined by the thickness of the bed or beds, their angle of inclination, and the slope or contour of the ground. Among a series of vertical strata the breadth of the outcrop of a bed corresponds exactly with the true thickness of that bed. The more the angle of inclination lessens, the broader does the outcrop at the surface become. Hence, in tracing such a band across a country, attention must constantly be given to the variations of angle in the dip. Where the dip increases the band narrows in breadth ; where the dip lessens the band widens. This is best seen on level or gently undu- lating ground 3 it is apt to be less distinctly shown where the ground is very uneven, and where therefore constant modifications of the line of outcrop are produced, as we have seen to be the case with horizontal strata. When strata are vertical no amount of surface irregularity makes any difference on their outcrop. They are apt in that position to run on for some distance with little deviation of direction, so that the outcrop of one of them might be marked by a straight bar or line (Fig. 26). The CH. ix.] GEOLOGICAL STRUCTURE. 103 influence of the form of the ground tells more and more upon the outcrop in proportion as the strata approach the horizontal. Strike. The outcrop of a stratum is the line which that stratum makes with the surface of the ground. This term "outcrop "is often spoken of as if it were the same as the "strike." The latter word is applied to a line drawn perpendicular to the direction of dip. It is the line made by a stratum with the horizon, and shows the general or average direction of that stratum across the country. On a perfectly level piece of ground strike and outcrop must obviously coincide, and there must likewise be a complete coincidence among vertical strata. The more irregular the surface, and the less inclined the strata, the further must strike and out- crop depart from each other. Relation of Strike to Dip. There is a further relation to be noted as we proceed, viz. the constant depend- ence of the direction of strike upon that of dip, and the consequent changes of strike as the direction of dip varies. The strike is of course a mathematical line cutting the dip at a right angle. If the dip is east or west, the strike must be north and south; if the dip is north or south, the strike must be east and west. It must not be supposed, however, that the line of strike is always, or even most commonly, a straight one. It can only be so as long as the direction of dip continues unchanged. But a comparatively brief experience in the field suffices to show how constantly the dip of strata varies, now to one side, now to another, every such variation producing a corresponding change upon the line of strike. Where the deviations are slight, and of 104 OUTLINES OF FIELD-GEOLOGY. [CHAP. local character, while the mean direction of inclination remains -the same, we take that mean direction as governing the strike (as at band F in Fig. 27). Where, on the other hand, the dip is to different points of the compass in succession over wide spaces, we connect the arrows on our map by lines (as in band Gin Fig. 27), and find that the strike becomes a curved, and even, it may be, a very sinuous one. Difference between Outcrop and Strike. These two terms ought to be distinguished, otherwise, in con- structing a geological map, we shall either lose the im- pression of the external form of the ground, which a correctly-traced outcrop so often vividly conveys, or we shall be in danger of regarding the dip as constantly changing, and the strata, though perhaps nearly flat, as extensively disturbed. Looking at any good geological map of England and Wales, that by Sir Andrew C. Ramsay, for example, the reader will notice that the bands of the Oolitic and Cretaceous rocks, while retain- ing a tolerably persistent strike from south-west to north east, across the breadth of the country, present a most sinuous and irregular edge. The direction of the dip, and consequently the trend of the strike, change but little, yet it will be observed that the outcrop is con- tinually shifting to-and-fro. The strata really follow each other in parallel bands. If we could plane down the whole country to a dead level, these bands would be marked by alternate strips of clays, limestones, and sandy rocks. But instead of being a flat, the country undulates, and hence a series of gently inclined rocks of very various degrees of durability necessarily gives a diversified set of outcrops. No better illustration could ix.] GEOLOGICAL STRUCTURE. 105 be studied of the difference between outcrop and strike, and of the marked influence even of small ridges and hollows and shallow valleys upon the outcrop of strata, where the angle of dip is low. The main facts to be expressed upon the map of such a tract of country are, that the formations follow each other in a certain order, and cross the region in a certain direction. Of course we might record these facts by simply drawing straight parallel strips across the map, each marking the position and relative breadth of one of the formations. This was the way in which the old geological maps on a small scale were constructed. The map of England and Wales in Bakewell's Geology, even so late as the edition of 1838, may serve as an example. But by such a style of .mapping we entirely lose, as I have just said, one of the valuable features of a geological map the relation between the form of the ground and the nature and out- line of the rocks below, that is, between scenery and geological structure. It may readily be believed that this is too important a relation to be ignored without great disadvantage when the scale of the map at all permits it to be expressed. Besides, the omission deprives the map of the chief feature by which the skilled and artistic observer is distinguished from him whose eye and hand are less quick to seize upon and delineate the characteristic varieties of form which geolo- gical boundaries assume as the surface of a country changes from plain to hill, and as the rocks themselves alter in thickness and position. Numerous illustrations of the applicability of this prin- ciple will occur to every observer in the field. If, for ex- ample, he stands at the higher margin of a rocky valley, 106 OUTLINES OF FIELD-GEOLOGY. [CH. ix. along the sides of which inclined beds of sandstone, limestone, or other stratified rocks are exposed, dipping gently down the valley, he observes that when the out- crop of each bed reaches the edge of the declivity it does not go straight on to the corresponding outcrop on the opposite side. On the contrary, it descends the slope in a slant until it reaches the bottom of the valley, when it turns and mounts the opposite slope, thus form- ing a V-shaped indentation on the general line of strike (as in the valleys on the south side of the map, Figs. 26 and 27). Now the manner in which these windings of the outcrop of inclined strata and their relation to the form of the ground are expressed upon the geological map is a good test of the skill and delicacy which I have insisted upon as so desirable in the map-work of a field-geologist. Many observers are content to draw the lines of out- crop as straight bars across the valleys, thus making them coincident with the strike. On maps of a small scale, indeed, nothing else is possible. But where the scale admits of it, much advantage may be gained by faithfully depicting the curving outcrops. The map then tells its story at once, and brings the relation between geological structure and external form as vividly before the eye as a well-made model could do. Construction of a Geological Map. In order to show the application of the foregoing remarks, two diagrams (Figs. 26 and 27) are given. In Fig. 26 an attempt is made to convey some idea of the way in which the data are compiled and recorded in the construc- tion of a geological map. The shaded parts of that figure represent what is actually seen by the geologist : over the blank portions he is supposed to have been loS OUTLINES OF FIELD-GEOLOGY. [CHAP. unable to find any rock in situ. Fig. 27 shows the map as filled in and completed from these data. I shall have occasion to make frequent references to these maps in what follows. It will be noticed that most of the observations occur along the stream-courses, these being the most frequent natural lines of section. At each point where the dip of strata has been taken, an arrow and number mark the direction and angle. The more important or strati- graphically serviceable beds have their outcrop marked in decided lines where it is actually seen. When the same stratum can be recognised in two parallel or adjacent streams or valleys, the outcrop may be drawn across the intervening ground, which of course should itself be searched for traces of the desired line. Where there can be no doubt as to the direction and position of the out- crop, it may be drawn as a continuous line or band. Where, however, though it is known to occur within certain limits, some doubt may exist as to its exact position, it should be expressed by broken or dotted lines. Establishing a Stratigraphical order of Succession. It will be seen from the map that in the streams at the lower part of the left side, the same beds are recognisable, following and dipping under each other at corresponding intervals. In other words, the order of succession is found to be the same in the different streams. Bed A after an interval is followed by bed , bed B by bed C, and so on. Even, therefore, where a blank space occurs, and, owing to some surface accumulation, a particular bed may not be visible in one of the lines of section, we can be tolerably sure of the place where, judging from the strata above and below, it would be seen if it ix.] GEOLOGICAL STRUCTURE.. 109 came to the surface. We do not hesitate, therefore, to draw dotted lines across that place to indicate our belief A geological map is thus derived partly from what is seen, and partly from what can be legitimately inferred. I would further direct attention to the fact that while the order in which the beds occur remains the same in all the streams upon our map (Figs. 26 and 27), the spaces between them vary considerably. This difference may arise from one or other of three causes; either (i) vari- ation in angle of dip, or (2) variation in thickness of strata, or (3) inequalities in the level of the ground. We have already considered the effect of a decrease of in- clination in increasing the breadth of a stratum or series of strata at the surface of the ground. It is further evi- dent that if the mass of strata between two known beds should swell out or diminish, the breadth of the space between their respective outcrops must correspondingly vary. Inequalities of the surface must likewise influence, as we have seen, not only the direction of the outcrops, but also their breadth. Where, therefore, the angle of dip does not change, and the surface of the ground presents no marked inequalities, but where, nevertheless, a decided widening or narrowing of the interval between two outcrops occurs, we infer with confidence that the intermediate strata must increase or diminish in thickness. Estimation of Thickness of Strata. When the angles of dip have been observed along a line perpendicular to the strike, it is easy to calculate what the thickness of rock must be in any given interval of the section, or to obtain it by using the protractor. This is most conve- niently done at home, where the observer can collect his notes and protract the angles he has taken in the field. OUTLINES OF FIELD-GEOLOGY. [CHAP. It is useful, however, to have a ready means of estimat- ing thickness, and in this respect the following rule, given by Mr. Charles Maclaren, 1 will be found of service. If the breadth of inclined strata is measured across their outcrop at right angles to the strike, their true thickness will be equal to T Vth of their breadth at the surface for every 5 of dip. Or it may be put thus : divide 60 by the angle of dip, and the fraction is obtained which ex- presses the thickness. Thus, suppose a mass of strata measures across the strike 1,200 feet, and is uniformly inclined at an angle of 5, its real thickness will be y^th, or 100 feet; at 10 the thickness will be ^th, or 200 feet; at 15 it will be ^th, or 300 feet; at 20, |rd, or 400 feet This rule is very nearly accurate for inclina- tions up to 45. Thinning away of Strata. Overlap. It sometimes happens that two lines of outcrop come together, owing to the complete thinning away of the intermediate strata, and the conjoined outcrops may then be traceable for a long distance without further change. Instances of this FIG. 28. Section of an overlap. kind sometimes occur among the coal-seams of coal- fields. The higher portions of a series of strata now and then steal over the lower, so as to constitute what is termed an "overlap." This structure cannot always be 1 Maclaren's Geology of Fife and the Lothians, p. xv. ix.] GEOLOGICAL STRUCTURE. m expressed in plan upon a map, but is made clear by a section. On the map (Fig. 27) the upper portions of the group Em overlaps upon group D in the neighbourhood of the locality marked O. A section of this part of the district would be as in Fig. 28. This structure may frequently be met with along the margins of formations deposited in tracts which were undergoing gradual submergence. As the land sank, successive zones were carried down beneath the sea, and the later deposits of the sea-floor were prolonged further and further beyond the limits of the earlier ones. FIG. 29. Overlap and unconformability, Mendip Hills (De la Beche). c, Old Red Sandstone: b, Lower Limestone Shale \a,a. Carboniferous Limestone: d, beach deposits of Lower Jurassic age passing up into e Lias ; f, Sands of Inferior Oolite, which are overlapped by g n. Inferior Oolite ; h, I, clay, and i, limestone of Fuller's earth. The accompanying section (Fig. 29) shows very clearly an overlap among the Jurassic beds, all of which lie un- conformably on the palaeozoic rocks. Unconformability. In an overlap the strata are parts of one continuous unbroken series, the formation of which does not appear to have been interrupted by any great physical disturbance, nor even in many cases by marked change of any kind in the general conditions of deposit. But where the accumulation of a group of rocks has been succeeded by its elevation, exposure, and denudation, the next set of rocks laid down upon it are said to lie unconformably ; and this kind of junction 112 OUTLINES OF FIELD-GEOLOGY. [CHAP. is termed an unconformability. Thus in Fig. 29 the Secondary formations (d /) lie unconformably upon the Palaeozoic rocks (a c). It is not necessary that the older rocks should have been disturbed from then- original horizontality they may have been equally upraised, and, after being exposed to denudation, may have been equally depressed again. It usually happens, however, that some tilt has been given to them, and consequently that the overlying rocks rest transgressively upon their upturned and worn edges. An unconformable junction is of the highest import- ance in the geological structure of a district. It marks one of the great gaps or intervals in geological history. The observer ought to spare no pains to collect all the available data in every case where he has reason to suspect the existence of such a structure. An extreme case presents little difficulty. It can be expressed so clearly upon a map as at once to tell its own story. Thus in Fig. 27 the sheet of rock H evidently forms a flat unconformable cake lying upon inclined and denuded strata. A section across this cake would disclose an abrupt junction of the horizon- tal beds on the edges of the steep and vertical series But many cases occur where the discordance between the two series is far less strong, where indeed much care and labour may be required to make out an un- conformability at all. For here again, as in the case of faults, the actual line of contact between the two groups of rocks is comparatively seldom seen. We must usually infer from their general arrangement and their relations to each other whether or not they are separated by an unconformability. In the diagram we have already ix.] GEOLOGICAL STRUCTURE. 113 used so much (Fig. 27) another and less violent uncon- formability is shown towards the north-west corner, where the series of beds K steals over the denuded outcrops of the series I. In collecting evidence on the subject of a supposed unconformability the observer should endeavour to realize to himself what must have been the contour of the ground at the time when the overlying rocks were ac- cumulated. He may in this way sometimes be led to see that his suspected unconformability is extremely un- likely, or physically impossible. As a curious illustration FIG. 30. Portion of a geological section with an impossible unconformability. of the consequences of the want of this precaution Fig. 30 is here inserted from a recently published geological section. The horizontal distance represented is about six miles, and as the upper rocks are made to dip at angles of between 40 and 50, there must be a mass of them somewhere about four and a half miles thick. If we suppose them to have been originally perfectly hori- zontal, they must have been laid down against the slopes of a mountain about four and a half miles high ; or if they sloped gently away from the underlying rocks, the height of the mountain must have been still greater. But not only, however, must a stupendous mountain have been tilted round so as to lie on its side, but the whole of the later rocks must have been removed except a u 4 OUTLINES OF FIELD-GEOLOGY. [CHAP. narrow cake cut across the bedding parallel with the original slope of the mountain. In reality there are not two sets of rocks in the line of section. The whole is one, subject here and there to local crump- ling, as might have been seen by more careful and extended observation. In most cases it is possible so to express an uncon- formable junction upon the map as to make it readily apparent to the geologist. It should be the aim of the surveyor to neglect no item of evidence which will enable him to do this ; for the more perfectly his map is self-in terpreting, the more useful will it be. Hence where, as is often the case, the ground is obscured by surface-accumu- lations, and a little liberty of choice is left to him as to the precise course along which to place his line of uncon- formability, he will draw his line in such a way as to show as clearly as may be that it is not a fault or an ordinary conformable junction. In some districts, particularly in those where older for- mations are covered by more recent superficial accumu- lations, a double unconformability may often be seen. The accompanying diagram, for example (Fig. 31), re- presents what is exposed in a cliff section at Cullen, on the coast of BamTshire. The lowest formation consists of quartz-rock ( and ( I Varieties 1 '.. Abundant among both ancient and mo- dern crystalline rocks, as granite, gneiss, quartz-porphyry, liparite, trachyte, ob- sidian, &c. - ^ Oligoclase j Albite / [. Abundant as the triclinic felspars of older crystalline rocks, as in granite, syenite, &c. |;| H Leuci Anorthite "1 Labradorite J te . . . . '. Abundant, more especially among vol- canic rocks of all ages from palaeozoic up to recent ; found also among ancient foliated rocks, as the gneiss of Labrador and in some granites. I. An essentially volcanic mineral, only found in lavas and tuffs of later geo- logical periods. g 1 Nepheline Hauyne .... [. In some volcanic rocks in minute prisms ; also massive in metamorphic rocks. [. Only found in post-tertiary lavas. f Nosean .... [I. Like Hauyne, a volcanic mineral of late geological date, said to occur in almost all phonolites. Muscovite (Potass-mica) [. Abundant in old crystalline rocks, both massive and foliated granite, gneiss, mica-schist, greisen, &c. ; also in sand- stones of all ages. Biotite .... (Magnesia-mica) I. Abundant in many crystalline rocks. II. As an occasional alteration-product in many hornblendic and augitic rocks. Chias Kyan tolite . . . ite I. Abundantly diffused through some me- tamorphic slates, hence called chiastolite- slates. T In small granular forms in some crystal- line rocks, e.g. kyanite-rock ; also in beds and veins in gneiss. Tourmaline . . I. In some granites, and in the veins asso- ciated with and proceeding from these granites. CHEMICAL TESTS. 197 Essential Constituent. II. Accessory Ingredient. C/2 O s t o -5 K J3 Garnet . Epidote . Cordierite (lolite) Zeolites . B < Ottrelite . Kaolin . . Hornblende Augite . Diallage . Abundant in many foliated rocks, as in mica-schist and some varieties of gneiss. . In some foliated rocks, as gneiss and mica schist. I. As an alteration-product in many rocks, as in diorite, diabase, altered sandstone, &c. . In geodes and veins among older crys- talline rocks, as granite, gneiss, and several schists ; one variety, cordierite- gneiss, contains it abundantly. I. This interesting family of minerals is due to the alteration of anhydrous alu- minous silicates, chiefly felspars. The several species occur as secondary pro- ducts in veins and cavities of rocks, es- pecially of such as contain abundant felspar. The amygdaloidal cavities of basalts and other basic volcanic rocks furnish many varieties. !. In some varieties of slate, as that of Ottrez, Luxembourg, whence the name ottrelite slate. [I. Decomposed felspar, apt to occur wher- ever a felspar- bearing rock is exposed to a moist climate. [. Abundant as a constituent of many mas- sive crystalline rocks, e.g. syenite and diorite ; of many foliated rocks, as horn- blende-schist, and varieties of gneiss. I. The black variety abundant among vol- canic rocks basalt, diabase, &c. ; the paler kinds among granitic and foliated rocks and not uncommon among crystal- line limestones. I. One of the constituents of gabbro ; also found in serpentine and hypersthene rock. I 9 8 OUTLINES OF FIELD-GEOLOGY. [CHAP. I. Essential Constituent. II. Accsssory Ingredient. 'Enstatite . . [. One of the constituents of the rock called Lberzolite ; occurs also in some serpentines and in some meteorites. Bronzite . [. In some serpentines and basalts ; also in some meteorites. 1? ' ^ s < Uralite . . . [I. A mineral having the crystalline form of augite but the internal fine fibrous character of hornblende, with sometimes a central core of still unaltered augite. In some old porphyritic rocks Urals, Norway, Alps, Scotland. Smaragdite "Talc .... I. A constituent of the rock called eclogite and some forms of gabbro. T Abundant among foliated rocks, some of which (e.g., talc-schist) consist largely of it. [I. As an alteration-product among crys- talline rocks. Chlorite . . . I. Constituting almost the whole of the rock termed chlorite-slate, and found among other foliated and crystalline rocks. [I. Frequent as an alteration-product in rocks containing hornblende, augite, oli- vine, or other anhydrous magnesian silicate. i % Serpentine . . [. Constitutes entire rock-masses, but these appear in all cases to have been origi- nally anhydrous, often olivine-rocks. II. Frequent as an alteration- product, par- ticularly in rocks containing olivine. Delessite . 1 Saponite . ^ (Celadonite) J II. Alteration-products in crystalline rocks, especially volcanic masses rich in mag- nesian silicates ; occur in kernels filling cavities, as incrustations, round nodules, or in irregular veinings and blotches. Glauconite . . (Silicate of Iron and Potass.) II. Abundantly diffused through soijidand- stones, and found also as an alteration- product among many old augitic and hornblendic volcanic rocks, lining cavi- ties or running in threads through the altered mass. XVI.] CHEMICAL TESTS. 199 I. Essential Constituent. II. Accessory Ingredient. rCalcite .... T Common as limestone or the calcareous constituent of stratified rocks. II. Very abundant as an alteration-pro- duct, filling cavities or running in veins and threads through rocks. 8 Aragonite . . . II. Under similar circumstances as calcite, 5 but less abundant. ; Dolomite .... T Occurs in beds and layers with lime- stone, red marl, sandstone, &c. <; II. In veins and cavities and along the CJ edges of intrusive igneous rocks. Siderite .... I. In beds and nodules associated with shale, coal, &c. II. Occasionally in veins and cavities of rocks. (Barytes . . . II. Common as a veinstone ; also found in cavities of amygdaloidal rocks. Celestine . . II. In cavities and veins in limestone, sandstone, and in some old volcanic 1*1 rocks. c w : < Anhydrite . . I. In beds, associated with limestone, red H < I sandstone, or rock-salt. S ( PH K) (Gypsum I. In beds with red strata, rock-salt, &c. t3 c/: ' II. In veins and strings through different rocks. ^ "j Alums . . . * II. Aluminous rocks, containing iron sul- phides, exposed to weathering are apt to decompose, and various alum-salts ap- k pear as an efflorescence. Apatite .... T Abundant in some metamorphic regions, both in layers and in veins ; an essential constituent of many crystalline rocks, as varieties of granite, diorite, diabase, H gabbro, and dolerite. < s II. Common as a veinstone in some metal- E liferous districts. Vivianite .... 11. In veins associated with metallic ores ; also as a blue earth in bogs and other places where animal remains have de- cayed, and as a peach-bloom on some ichthyolites. OUTLINES OF FIELD-GEOLOGY. [CH. xvi. I. Essential Constituent. II. Accessory Ingredient. Sphene . . Asphalt . . Naphtha . . Anthracite . , Abundant in some granites, syenites, gneisses, schists, and metamorphic lime- stones. II. Occasionally disseminated in grains or filling veins and cavities of sandstone or other rocks. II. Occasionally in cavities of rocks, or coming to surface either alone or with spring water. In beds, like ordinary coal. II. In cavities of rocks, particularly in as- sociation with intrusive igneous masses ; also diffused in minute grains, giving a black, coally aspect to some rocks. CHAPTER XVII. MICROSCOPICAL INVESTIGATION. FREQUENT reference has been made in the foregoing pages to the advantage of studying minerals and rocks under the microscope. By this means we are enabled to trace the minuter structures of the earth's crust, and to follow many of the stages in the formation of its rocks. We can tell which mineral of a rock crystallized first, and indeed can follow every phase of crystallization, in such a way as to explain many otherwise unknown parts of the history of the rocks. Moreover by this method we can trace the subsequent changes which rocks have suffered, in the chemical alteration of their minerals by percolating water, with the resulting secondary products. While a chemical analysis informs us of the ultimate chemical constitution of a rock, a microscopic analysis brings before us its mineralogical composition, showing in what forms the chemical elements have been combined, and how diverse two rocks may be in structure and texture, though in chemical composition nearly alike. The field-geolcgist, however, besides these inquiries, often needs some ready means of determining the nature and petrographical grade of rocks which he cannot 202 OUTLINES OF FIELD-GEOLOGY. [CHAP. satisfactorily name by any of the usual methods available to him. By far the most valuable aid in this respect is supplied to him in the examination of thin slices with the microscope. He ought to be able to prepare his own slices, though when he can have this satisfactorily done for him he may save time for other work. THE PREPARATION OF THIN SLICES. To prepare slices of rocks and minerals for the micro- scope it is not necessary to procure a costly and unwieldy set of apparatus, nor to go through a lengthened course of training. The operation is facilitated, indeed, by the possession of a machine for cutting thin slices, and for reducing and polishing them when mounted on glass. A machine well adapted for both purposes was devised some years ago by Mr. J. B. Jordan, and may be had of Messrs. Cotton and Johnson, Grafton Street, Soho, London, for 10 los. Another slicing and polishing machine invented by Mr. F. G. Cuttell, 52, New Compton Street, Soho, London, costs 6 los. But these machines are rather unwieldy to be carried about the country by a field-geologist. Fuess of Berlin supplies two small and convenient hand-instruments, one for slicing, the other for grinding and polishing. The slicing machine is not quite so satisfactory for hard rocks as one of the larger more solid forms of apparatus worked by a treadle. But the grinding-machine is ex- ceedingly useful, and might be added to a geologist's outfit without material inconvenience. If a lapidary is within reach, much of the more irksome part of the work MICROSCOPICAL INVESTIGATION. 203 may be saved by getting him to cut off thin slices. The thickness of each slice must depend greatly upon the nature of the rock, the rule being to make the slice as thin as the rock will allow, so as to save labour in grinding down afterwards. Excellent rock-sections, however, may be prepared without any machine, provided the operator possesses ordinary neatness of hand and patience. He must pro- cure as thin chips as possible of the rocks he proposes to slice. These he can usually obtain in the field where he is hammering. He should select as fresh a portion of the rock as may be accessible, and by a dexterous use of the hammer break off from a sharp edge a number of thin splinters or chips, out of which he can choose one or more for making into rock-slices. These chips may be about an inch square. It is well to take several of them, as the first specimen may chance to be spoiled in the preparation. The geologist ought also always to carry off a piece of the same block from which his chip is taken, that he may have a specimen of the rock for future reference and comparison. Every such hand- specimen, as well as the chips belonging to it, ought to be wrapped up in paper on the spot where it is obtained, and inside the wrapper should be placed a label or piece of paper with the locality and any notes that may be thought necessary. It can hardly be too frequently reiterated that all such field-notes ought as far as possible to be written down on the ground where the actual facts are before us for examination. Having obtained his thin slices, either by having them slit with a machine or by detaching with a hammer as thin splinters as possible, the operator may proceed 204 OUTLINES OF FIELD-GEOLOGY. [CHAP. to the preparation of them for the microscope. For this purpose the following simple apparatus is all that is absolutely needful, though if a grinding-machine be added it will save time and labour. List of Apparatus Required in the Preparation of Thin Slices of Rocks and Minerals for Microscopical Ex- amination. 1. A cast-iron plate, | inch thick and 9 inches square. 2. Two pieces of plate-glass, 9 inches square. 3. A Water- of- Ayr stone, 6 inches long by 2^ inches broad. 4. Coarse emery (l Ib. or so at a time). 5. Fine or flour emery (ditto). 6. Putty powder (i oz.). 7. Canada balsam. (There is an excellent kind prepared by Rimmington, Bradford, especially for microscopic preparations, and sold in shilling bottles.) 8. A small forceps. 9. Some oblong pieces of common flat window-glass 52X1 inches is a convenient size. 10. Glasses with ground edges for mounting the slices upon. They may be had at any chemical instrument-maker in different sizes, the commonest being 3X1 inches. 11. Thin covering- glasses, square or round. These are sold by the ounce ; \ ounce will be sufficient to begin with. 12. A small bottle of spirits of wine. The first process consists in rubbing down and polishing one side of the chip or slice. We place the chip upon the wheel of the grinding-machine or, failing that, upon the iron plate, with a little coarse emery and water. If the chip is so shaped that it can be conveniently pressed by the finger against the plate and kept there in regular horizontal movement, we may proceed at once to rub it xvn.] MICROSCOPICAL INVESTIGATION. 205 down. If, however, we find a difficulty, from its small size or otherwise, in holding the chip, one side of it may be fastened to the end of a bobbin or other convenient bit of wood by means of a cement formed of three-parts of rosin and one of bees-wax, which is easily softened by heating. A little practice will show that a slow, equable motion with a certain steady pressure is most effectual in producing the desired flatness of surface. When all the roughnesses have been removed, which can be told after the chip has been dipped in water so as to remove the mud and emery, we place the specimen upon the square of plate-glass, and with flour-emery and water continue to rub it down until all the scratches caused by the coarse emery have been removed and a smooth polished surface has been produced. Care should be taken to wash the chip entirely free of any grains of coarse emery before beginning to the polishing on glass. It is desir- able also to reserve the glass for polishing only. The emery gets finer and finer the longer it is used, so that by remaining on the plate it may be used many times in succession. Of course the glass itself is worn down, but by using alternately every portion of its surface and on both sides, one plate may be made to last a considerable time. If after drying and examining it carefully, we find the surface of the chip to be polished and free from scratches, we may advance to the next process. But it will often happen that the surface is still finely scratched. In this case we may place the chip upon the Water-of- Ayr stone and with a little water gently rub it to and fro. It should be held quite flat The Water-of-Ayr stone too should not be allowed to get worn into a hollow, but should be kept quite flat, otherwise we shall lose part 206 OUTLINES OF FIELD-GEOLOGY. [CHAP. of the chip. Some soft rocks, however, will not take an unscratched surface even with the Water-of-Ayr stone. These may be finished with putty-powder, applied with a bit of woollen rag. The desired flatness and polish having been secured, and all trace of scratches and dirt having been completely re- moved, we proceed to grind down the opposite side and reduce the chip to the requisite degree of thinness. The first step at this stage is to cement the polished surface of the chip to one of the pieces of common glass. A thin piece of iron (a common shovel does quite well) is heated over a fire, or is placed between two supports over a gas-flame. On this plate must be laid the piece of glass to which the specimen is to be affixed, and the specimen itself. A little Canada balsam is dropped on the centre of the glass and allowed to remain until it has acquired the necessary consistency. To test this condition, the point of a knife should be inserted into the balsam, and on being removed should be rapidly cooled by being pressed against some cold surface. If it soon becomes hard it has been sufficiently heated. Care, however, must be observed not to let it remain too long on the hot plate; for it will then become brittle and start from the glass at some future stage, or at least will break away from the edges of the chip and leave them exposed to the risk of being frayed off. The heat should be kept as moderate as possible, for if it becomes too great it may injure some portions of the rock. Chlorite, for ex- ample, is rendered quite opaque if the heat is so great as to drive off its water. When the balsam is found to be ready, the chip, which has been warmed on the same plate, is lifted with the xvii.] MICROSCOPICAL INVESTIGATION. 207 forceps and its polished side is laid gently down upon the balsam. It is well to let one end touch the balsam first, and then gradually to lower the other, as in this way the air is driven out. With the point of a knife the chip should be moved about a little, so as to expel any bubbles of air and promote a firm cohesion between the glass and the stone. The glass is now removed with the forceps from the plate and put upon the table, and a lead weight or other small heavy object is placed upon the chip, so as to keep it pressed down until the balsam has cooled and hardened. If the operation has been successful the slide ought to be ready for further treatment as soon as the balsam has become cold. If, however, the balsam is still soft, the glass must be again placed on the plate and gently heated, until on cooling the balsam resists the pressure of the finger-nail. Having now produced a firm union of the chip and the glass, we proceed to rub down the remaining side of the stone with coarse emery on the iron plate as before. If the glass cannot be held in the hand or moved by the simple pressure of the fingers, which usually suffices, it may be fastened to the end of the bobbin with the rosin cement as before. When the chip has thus been reduced until it is tolerably thin, until, for example, light begins to appear through it when held between the eye and the window, we may, as before, wash it clear of the coarse emery and continue the reduction of it on the glass plate with fine emery. Crystalline rocks, such as granite, gneiss, diorite, dolerite, and modern lavas, can be reduced to the required thinness on the glass. Softer rocks may require gentle treatment with the Water -of- Ayr stone. 2o8 OUTLINES OF FIELD-GEOLOGY. [CHAP. The last parts of the process are the most delicate of all. We desire to make the section as thin as possible, and for that purpose continue rubbing until after one final attempt we perhaps find to our dismay that great part of the slice has disappeared. The utmost caution must consequently be used. The slide should be kept as flat as possible, and looked at frequently, that the first indications of disruption may be detected. The thinness desirable or attainable depends in great measure upon the nature of the rock. Transparent minerals need not be so much reduced as more opaque ones. Some minerals, indeed, remain absolutely opaque to the last, like pyrite, mag- netite, and ilmenite. The slide is now ready for the microscope. It ought always to be examined with that instrument at this stage. We can thus see whether it is thin enough, and if any chemical tests are required they can readily be applied to the exposed surface of the slice. If the rock has proved to be very brittle, and we have only succeeded in pro- curing a thin slice after much labour and several failures, nothing further should be done with the preparation unless to cover it with glass, as will be immediately explained, which not only protects it, but adds to its transparency. But where the slice is not so fragile, and will bear removal from its original rough scratched piece of glass, it should be transferred to one of the glass-slides (No. 10). For this purpose the preparation is once more placed on the warm iron plate, and close alongside of it is put the glass-slide, which has been carefully cleaned, and on the middle of which a little Canada balsam has been dropped. The heat gradually loosens the cohesion of the slice, which is then very gently pushed MICROSCOPICAL INVESTIGATION. 209 along to the contiguous clean slip of glass. Considerable practice is needed in this part of the work, as the slice, being so thin, is apt to go to pieces in being transferred. A gentle inclination of the warm plate is advantageous, so that a tendency may be given to the slice to slip downwards of itself on to the clean glass. We must never attempt to lift the slice. All shiftings of its posi- tion should be performed with the point of a long needle or other sharp instrument. If it goes to pieces we may yet be able to pilot the fragments to their resting place on the balsam of the new glass, and the resulting slide may be sufficient for the required purpose. When the slice has been safely conducted to the centre of the glass slip, we put a little Canada balsam over it, and allow it to be warmed as before. Then taking with the forceps one of the well-cleaned thin cover-glasses, we allow it gradually to rest upon the slice by letting down first one side, and then by degrees the whole. A few gentle circular movements of the cover-glass with the point of the needle or the forceps may be needed to ensure the total disappearance of air-bubbles. When these do not appear, and when, as before, we find that the balsam has acquired the proper degree of consistence, the slide con- taining the slice is removed, and placed on the table with a small lead weight above it in the same way as already described. On becoming quite cold and hard the super- abundant balsam round the edge of the cover-glass may be scraped off with a knife, and any which still ad- heres to the glass may be removed with a little spirits of wine. Small labels should be kept ready for affixing to the slides to mark the locality and reference number of the p 2io OUTLINES OF FIELD-GEOLOGY. [CHAP. specimen. Thus labelled the slide may be put away for future study and comparison. The whole process seems perhaps a little tedious. But in reality much of it is so mechanical that after the mode of manipulation has been learnt by a little experience, the rubbing-down may be done while the operator is reading. Thus in the evening, when enjoying a pleasant book after his day in the field, he may at the same time with some practice rub down his rock-chips, and thus get over the drudgery of the operation almost unconsciously. Boxes with grooved sides for carrying microscopic slides are sold in different sizes. Such boxes are most convenient for field-work, as they go into small space, and with the help of a little cotton-wool they hold the glass-slides firmly without risk of breakage. For a final resting-place, a case with shallow trays or drawers in which the slides can lie flat is most convenient. One final remark may here be required. The learner must not suppose that having prepared his slices, he has nothing to do but to place them under the microscope and at once determine their composition. He will find it by no means an easy task to make satisfactory progress, and at first he may be inclined to abandon microscopic work in despair of ever gaining confidence in it. Let him, however, begin by studying individual minerals, and make himself acquainted gradually with their various characters. He should procure numerous sections of minerals which enter into the composition of the rocks which he wishes to investigate. By degrees he will be able to discriminate them as they occur in the rocks, and once able to do this, his progress will be compara- tively smooth. But he must be prepared for a long xvn.] MICROSCOPICAL INVESTIGATION. 211 patient course of training, and ought on no account to speak confidently about the microscopic structure of rocks until he feels assured that the confidence arises from sound knowledge. THE MICROSCOPE. As already stated (ante, p. 30), it is not necessary to procure an expensive microscope with veiy high magni- fying powers. For most purposes of the field-geologist the i^-inch objective with a magnifying power of from 20 to 50 or 60 diameters, according to the eye-pieces employed, will be found the most generally useful. But he should also have an objective capable of giving, with suitable eye- piece combinations, magnification up to from 200 to 300 diameters. A nose-piece for both objectives screwed to the foot of the tube saves much time and trouble by enabling the observer at once to pass from a low to a high power. Two Nicol prisms are indispensable ; one of these is to be fitted below the stage, the other is most advantageously placed over the eye-piece. A quartz- plate is useful. It should be so arranged below the stage as to be conveniently slipped in and out of the field as required. The numerous small pieces of apparatus necessary for physiological work are not needed in the examination of rocks and minerals. Reflected Light. It is always desirable to observe the characters of a rock as an opaque object. This cannot usually be done with a broken fragment of the stone, except of course with very low powers. Hence one of the most useful preliminary examinations of a prepared slice is to place it in the field, and, throwing the mirror p 2 212 OUTLINES OF FIELD-GEOLOGY. [CHAP. out of gear, to converge as strong a light upon it as can be had, short of bright direct sunlight. The advantage of this method is more particularly noticeable in the case of opaque minerals. The sulphides and iron oxides so abundant in rocks appear as densely black objects with transmitted light, and show only their external form. But by throwing a strong light upon their surface we may often discover that they possess a distinct and character- istic internal structure. Titaniferous iron is an admirable example of the advantage of this method. Seen with transmitted light that mineral appears in black, utterly structureless grains or opaque patches, though frequently bounded by definite lines and angles. But with re- flected light the cleavage and lines of growth of the mineral can then often be clearly seen, and what seemed to be uniform black patches are then found in many cases to inclose bright brassy kernels of pyrite. Transmitted Light. It is, of course, with the light allowed to pass through the prepared slices that most of the microscopic examination of minerals and rocks is performed. A little experience will show the learner that in viewing objects in this way he may obtain some- what different results from two slices of the same rock according to their relative thinness. In the thicker one a certain mineral will appear perhaps brown or almost black, while in the other what is evidently the same mineral may be pale yellow or green, or almost colour- less. Dichroism. Some minerals show a change of colour when a Nicol prism is rotated below them. Hornblende, for example, exhibits a gradation from deep brown to dark yellow a mineral presenting this change is said to MICROSCOPICAL INVESTIGATION. 213 be dichroic. To ascertain the dichroism of any mineral we remove the upper polarizing prism and leave only the lower. If as we rotate the latter directly under the stage of the microscope no change of tint can be ob- served, there is no dichroic mineral present, or at least none which shows dichroism at the angle through which it has been cut. But we may often detect little crystals which offer a lively change of tone as the prism goes round ; these are examples of dichroism. This behaviour may be used to discriminate the mineral constituents of rocks. Thus the two minerals hornblende and augite in many respects resemble each other. They differ in their cleavage angles, but these cannot always be found in microscopic slices. Augite, however, remains passive or nearly so while the lower prism is rotated : it is not dichroic, or only very feebly so. Hornblende, on the other hand, is very strongly dichroic. Polarized Light. By means of polarized light an exceedingly delicate method of investigation is made available. We use both the Nicol prisms. If the object is a piece of glass, or an amorphous body, or a crystal belonging to some substance which crystallizes in the regular or cubic system, the light will reach our eye unaffected by the intervention of the object. The field will remain dark when the axes of the two prisms are at right angles, in the same way as if no intervening object were there. If however, the substance under examination is a mineral belonging to one of the other crystallographic systems, it will modify the polarized beam of light. On rotating one of the prisms we perceive bands or flashes of colour, and numerous lines appear which before were invisible. The field 2i 4 OUTLINES OF FIELD-GEOLOGY. [CHAP. no longer remains dark when the two Nicol prisms are crossed. It is evident, therefore, that we may readily tell by this means whether or not a rock contains any glassy con- stituent. If it does, then that portion of its mass will become dark when the prisms are crossed, while the cr) stalline parts will remain conspicuous by their bright- ness. A thin plate of quartz makes this separation of the glassy and crystalline parts of a rock even more satisfac- tory. It is placed under the stage, and the Nicol prisms are so adjusted with reference to it that the field of the microscope appears uniformly violet. The glassy portion of any rock placed on the stage will allow the violet light to pass through unchanged, but the crystalline portions will show other prismatic colours. The object should be rotated in the field and the eye kept steadily fixed upon one portion of the slide at a time, so that any change may be observed. It would be far beyond the compass of this little hand- book to enter fully into the microscopic examination of rocks. The student who desires to pursue the subject further will find much assistance in the works quoted below. 1 For his satisfaction in the determination of rocks he may propound to himself the following questions : ist, Is the rock entirely crystalline, consisting solely of crystals of different minerals interlaced ; and if so, what are these minerals ? 2nd, Is there any trace of a glassy ground-mass? If there is he may remove the rock at once from the granite series. 3rd, Can he detect any evidence of the devitrification of what must have been 1 Sorby " On the Microscopic Struc'ure of Crystals, "indicating the Origin of. Minerals and Rocks," Quart. Jonrn. Gco. Soc. xiv. xvn.] MICROSCOPICAL INVESTIGATION. 215 at one time the glassy basis of the whole rock? This devitrification might be shown by the appearance of numerous microscopic hairs, rods, bundles of feather- like irregular or granular aggregations. 4th, In what order did the minerals crystallize ? This may often be very clearly made out with the microscope, as, for instance, where one mineral is completely inclosed within another. 5th, What is the nature of any altera- tion which the rock may have undergone ? In a vast number of cases the slices show abundant evidence of such metamorphism; felspar passing into a granular kaolin, augite changing into various indefinite green products termed " viridite," olivine into serpentine, while secondary calcite, quartz, and zeolites run in minute veins or fill up interstices of the rock. 6th, Is the rock a fragmental one ; and if so, what is the nature of its component grains ? Is any trace of organic remains to be detected ? In fine, I return once more to the main purpose of this book, which is to induce the reader to cultivate geology as an out-of-door recreation, and to give him a few hints for his guidance. Apart from its healthful mental training as a branch of ordinary education, 453; Zirkel's Mikroskopische Bescha/enheit der Mineralien und Gesteine (Leipzig, 1873) ; Rosenbusch's Mikroskofische Physiographic der Mineralien und Gesteine, 2 vols. (Stuttgart, 1873, 1876). [Since this note was in type an excellent manual on petrography by my friend Mr. F. Rutley, has been published by Messrs. Longman and Co.] 216 OUTLINES OF FIELD-GEOLOGY. [CH. xvn. geology as an open-air pursuit affords an admirable training in habits of observation, furnishes a delightful relief from the cares and routine of every-day life, takes us into the open fields, and the free fresh face of nature, leads us into all manner of sequestered nooks, whither hardly any other occupation or interest would be likely to send us, sets before us problems of the highest in- terest regarding the history of the ground beneath our feet, and thus gives a new charm to scenery which may be already replete with attractions. Even, therefore, should the reader never write a single sentence of geo- logical description, nor venture to put one geological line upon a map, he may gain from the prosecution of field-geology many a happy and profitable hour, alike in the country into which the pursuit leads him, and in his own home with quiet reflection on what he has seen and done in the field. INDEX. INDEX. Acid-bude, 27 Acid test f or minerals and recks, 188 Alb.te, 196 Alums, 199 Amygdaloidal texture, 57, 152 Anhydrite, 199 Anorthite, 196 Anthracite, 200 Anticlines, 134 Apatite, 190 Aragomte, 196 Artibt.c power in geological map- making, 96, 105 Asphalt, 200 Augite, 197, 213 Barytes, 199 Basalt intruded among carbonacecus rocks, 149 Basalt, weathering of, 52, 90, 188 Biot.te, 196 Black colours of rocks, 62 Blends, ic 4 Blowpipe apparatus, 29, 191 Blowpipe, use of, 190 Blue colour of rocks, 64 Buulder-clay, 172, 177 Boulders, 172, 177 Boundary-lines, geological, 87, 95 Breccia, 59 Brick earth. 169 Bronzite, 198 Brown colour of rocks, 63 Calcedony, 195 Calcite, 199 Canoes in peat-mosses, 168 Celadonite, 198 Celestine, 199 Cellular structure of rocks, 57 Chalcopyrits, 194 Chemical work of field -geolgist, 188 Chiastolite, 196 Chlorite, 198, 206 Climate, ind.cated by fossils, 75 Clinometer, 24, 97 Coal, formation of, 82 alteration of, by igneous rocks, 149 Coast-ice, 174 Compass, pocket, 23, 97 Conglomerate, 59, 73, 91 Contortion of rocks, at landslips, 136 regional, 137 Cordiente, 197 Cornwall, mining tracts of, 164 Crannoges, 169 Crystalline texture of rocks, 54 Curvature of rocks, 131 at landslips, 136 Delessite, 168 Detritus, rounded and angular, 91 Diallage, 197 Dichroism, 212 Dip of rocks, 97 Dolomite, 199 " Drums" of boulder-clay, 172 Dykes, 146, 147, 148, 153, 155 Earth-worm, geological operation tf, 89 Elevation, proved by fossils, 80 Enstatite, 198 Epidcte. 197 Erratic blocks, 172 Escarpments, 123, 173 Eskers, 172 INDEX Faults, nature of, 117 apt to be concealed at the surface, generally form no feature above Haematite, 195 Hammer, geological, 20 ground, 119 how detected and traced, 120, 126 Hardness of minerals and rocks, 60 Haiiyne, 196 hade or inclination of, 120 Hayden, Dr. F. V., method of survey- indicated by springs, 122 ing by, 37, 45 strata inverted by, 125 Highlands of Scotland, geology of, d'p-, 129 121, 128, 137, 160, 161 strike-, 129 Horizontal strata, how mapped, 94, contortions accompanying, 136 101 strata vertical at, 127 Hornblende, 197, 212 Field-geology, defined, 8, 18 first essays in. 10, 39, 48 accoutrement for, 19 I Fingal's Cave, 146 Fireclay, 59 Ice, transporting power of, 174, 177 Fluor-spar, 195 Icebergs, action of, 174 Foliation coincident with stratifica- Iceworn rock-surfaces, 174, 176 tion, 162 Igneous rocks, alteration effected by. Footprints, fossil, 78 148 Fossil, definition of, 70 alteration of, in contact with coal, Fossilization, conditions of, 71 149 Fossils, hunting for, n, 73 determination of, 141 often beget a taste for geology, n, 70 interbedded, 144, 150 interstratifications of, 145, 153 uses of, 12, 70, 75, 168 intrusive, 144, 146 plutonic, 144, 154 volcanic, 144 G sandstone, filling cracks in, 152 Galena, 194 Garnet, 197 Ilmenite, 195. 208, 232 Indoor geological work, 179 Inversion of strata, at faults, 125 Geographical conditions indicated by fossils, 76 Geology, Field, defined, 8, 18 in mountain chains. &c., 138 proved by fossils, 82 Iron, the great pigment in rocks. 62 practical knowledge of, how to be acquired, 7 surface, 166 J Geological horizons fixed by fossils, Geological horizons, importance of, 134 Joints, influence of, in scenery, if6 Geological map, definition of, 33 contractions used on, 38 K construction of, 106 Geological processes, 16 Geological Survey of Great Britain, 19, 21, 23, 28, 34, 38, 42, 130, 150, Kames (Eskers, Osar), 172 Kaolin, 197 Kyamite, 158, 196 154, 184 Glacier moraines, 172 Glaciers, transport by, 174 L Glaciati r n, 174 Glaucon.te, 198 Labradorite, 196 Gneiss, 159 Lake-bottoms, traces of, shown by Granite-veins, 155 fossils, 78 Granite, weathering of, 52, 67 Land-surfaces, indicated by fossil evi- Graphite, 194 Green colour of rocks, 64 dence, 77 Landslips, contorted strata at, 136 Grey colour of rocks, 65 Lens for field-geology, 23 Gypsum, 199 Leucite, 196 INDEX. Limestone in schist', i e o Limestone, crinoidal, 59, 82 weathering of, 52, 74, 90 Limonite, 195 Logan, Sir W. E., his geolog-'cal Sur- vey of Canada, 27, 36 London-basin, 183 M Magnetite, 193, 195, 208 Maps, geological, construction of, 36 importance of, in geology, 20, 31 influence of incorrect, 34 Marcasite, 194 Microscope, 30, 211 Microscopical investigation of rocks, 142, 143, 201 Mineralogy, uses of, 14, 49 Minerals, a guide to geological struc- ture, 157 identification of, 188 Mineral-veins, 162 how detected, 163 Mole, geological operation of, 89 Monoclinic felspar, 196 Moraines of glaciers, 172 Mud, 59 Muscovite, 196 N Necks or volcan ; c pipes, 146, 149, 154 Nepheline. 190, 196 Nosean, 196 Note-book, form and use of, 26 Oligoclase, 196 Ordnance Survey, maps of, 185 Orthoclase, 196 Osar(Eskers, Kames), 172 Ottrelite, 197 Outcrop, 100, 101, 103 Overlap, no Peat-mosses, 163 Pencils, use of coloured, in field-work, Petrography, 49 Plagioclase felspar, 196 Polarized light, 213 Porphyritic texture, 56 Psilomelane, 195 Pyrite, 194, 208 Quartz, 190, 195 Rabbits, geological operation of. 89 Rain-prints, occurrence of, among rocks, 77 Reconnaissances, geological, 39 Red colours of rocks, origin of, 63 Reflected light in microscopic research, Ripple-mark in rocks, 77 River-action as a geological operation, 76, 174 River-terraces, 170 Rocks, alteration of, by igneous intru- sion, 148 behaviour of, in mass, 67 cellular structure of, 151 chronology of, 83 cleavage of, 138 collecting of, 14, 18 colours of, 50, 62 crystalline, 54 curvature of, 130 determination of, 48, 141, 188, 202 effects of different, on scenery, 67 feel of, 62 fine-grained, 58 foliated, 161 fragmental, 58, 143 fresh fracture and weathering of. glassy, 57 hardness of, 60, 143 homy, 57 igneous, 141 inversions of, 137 joints of, 166 microscopical investigation of, 202 organically derived, 59 schistose, 57, 142, 156 slaggy structure of, 151 smell of, 62 stratigraphical succession of, 108 streak of, 60 structure and texture of, 54 Rock-salt, 195 Rock-slicing apparatus, 29 Sapomte, 198 Scenery, geological conditions of, 67, 105, 166 Schistose rocks, characters of, 57, 142. INDEX. Schistose rocks, foliation of, 159, 160 grits and conglomerates in, 162 investigation of, 157 limestone in, 160 minerals in, 158 Schistose texture of rocks, 57 Scoriaceous texture of rocks, 57 Scotland (see Highlands) Sea, action of, in geology, 16, 174 Sea, traces of former presence cf, proved by fossils, 13, 78 Sections, geological, construction of, Talc, 198 Terrestrial movements proved by fossils, 80 Thickness of rocks, how estimated, 109 Time in geology, 114 Topography of maps, 34 Tourmaline, 196 Transmitted light in microscopic re- search, 212 Triclinic felspars, 196 Tuffs, volcanic, 59, 143, 150, distorted, 183 A LUIS, vujuiuili;, 5y, 143, i^u, 1;,^ horizontal, 182 u natural, 87, 91 on true scale, 183 vertical, 182 Serpentine, 198 Unconf rmability, in, 113 United States, scenery and geology in, 66, 68 Shale, 59 Shale, alteration of, by igneous rocks. 148 Geological Survey of, 37, 45 Uralite, 198 Siderite, 199 v Sketching, value of, in field-geology, 40. 1 80 Slickensides, 175, 176 Smaragdite, 198 Veins, mineral, detection of, 162 Veins of igneous reck, 146, 147, 153, Smith, William, his geological work, 84 Soil, formation of, 88 Veinstones, 163 Vicissitudes in geology, 3, 85 Vivianite, 199 Sphene, 200 Springs, use of, in field-geology, 90, Volcanic cones, structure of, 153 122 Staffa, columnar basalt of, 146 W Stone implements as fossils, 168, 171 Wad, 195 Striated rock surfaces, 174, 176, 177 Strike of rocks, 103 Waves, action of, 16, 174 Weathering of rocks. 5, 51, 62, 74.. 141, Submerged forests, 81 Subsidence of land, proved by fossils, 1 88 White colour of rocks. 62 Worm-burrows and trails, 77 Subsoil, formation of, 88 Sulphides, 194, 208, 211 Y Sulphur, 194 Sun-cracked rock-surfaces, 78 Superficial deposits, 15, 168 Yellow colours of rocks. 63 Surface geology, 166 mounds, 171 Z Synclines, structure of, 134 form hills, 135 Zeolites, 197 RtCHARD CLAY & SONS, AD STREET HILL, LONDON, B.C. 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