Outlines of field-geology

Front matter

THE LIBRARY

OF

THE UNIVERSITY

Preface To The Second Edition

AT 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 ::geological maps and instruments of surveying. These "lectures formed part of a series designed for teachers, ! .and in illustration of the Loan Collection of Scientific • Instruments at that time exhibited. Treating ^the subject allotted to me in what seemed likely to prove the

most useful manner, I dwelt more specially upon the NS methods of observation requisite in ordinary field-geology ; and endeavoured to show how by the practice , of these methods geological maps and sections, repre-

X

senting in condensed form the facts established by

^N

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 permanent 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 thro\vn 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 unpretending elementary character of the original lectures. My 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 interested 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 l>e sought in quarries and ravines, by hillside and sea-shore. But hints regarding what should be looked for and 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 intelligently 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. PACK

I.— INTRODUCTORY I

PART L— OUT-OF-DOOR WORK.

Part Ii.—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 2gi

LIST OF ILLUSTRATIONS.

FIG. 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 S3

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 Lefidodendron. Carmarthenshire (De la Beche) . 60 xU LIST OF ILLUSTRATIONS.

FAGS

'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

16. Rain-prints in sandstone 7^

17. Sun-cracked surface of red sandstone marked with foot-

prints, Hildburghausen, Saxony .... 79

18. 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 ihe 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. PACK

26. Map showing the data from which a complete geological

27. Complete geological map 107

28. Section of an overlap no

29. Overlap and unconformability, Mendip Hills (De la

Beche) in

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 Watcher, 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

LIST OF ILLUSTRATIONS.

PAGK

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 138

46. Section of the grand inversions of strata in the Glarnisch

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 stratified 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 32. Section of a dyke rising through a small fault and sending 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 flattened 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

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 apparently 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 mountains, 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 S B 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 boulderstone 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 woodland orchards and cottages, a strong rib of rock, forming everywhere a marked feature, with its grey, lichencrusted 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 answering 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.

In the course of another walk we might stumble upon a hit 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 conglomerate 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 explanation, and, on the other, that he has to disabuse himself 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 interesting 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 mountains. 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 refuses 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 range the others in their orderly subordinate places, so that the harmony of the whole is seized, and the impression which it makes is fixed upon the mind. If I may be allowed to make the comparison, it is with the appreciation 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 geological 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 extensively 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 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 lfcp*4«usland 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 practical 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

!.] 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 advantage 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 effecis of floods, the evidence of successive deposits, and heightening of the flood-plain. Lut 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. n

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 fossiliferous, 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 proportion 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 amusement 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 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 these 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 region 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.