Introductory Text-book of Geology. Twelfth and enlarged edition. Revised and in great part rewritten by C. Lapworth

Front matter

WORKS ON NATURAL HISTORY. By HENBY ALLEYNE NICHOLSON, M.D., D.Sc, M.A., Ph.D., F.L.S., F.G.S., Regius Professor of Natural History in the University of Aberdeen. A MANUAL OF ZOOLOGY, for the Use of Students. With a General Introduction on the Principles* of Zoology. Sixth Edition, revised and greatly enlarged. Crown 8vo, pp. 865, with 454 Engravings on Wood, 14s. " It is the best manual of zoology yet published, not merely in EDgland, but in Europe."— Pall Mall Gazette. " The best treatise on zoology in moderate compass that we possess." — Lancet. TEXT -BOOK OF ZOOLOGY, for the Use of Schools. Fourth Edition, enlarged. Crown Svo, with 188 Engravings on Wood. 7s. titl. "This capital introduction to natural history is illustrated and well got up in every way. We should be glad to see it generally used in schools." — Medical Press and Circular.

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Introduction

TO THE STUDY OF BIOLOGY. Crown Svo, with numerous Engravings. 5s. A MANUAL OF PALÆONTOLOGY, for the Use of Students. With a General Introduction on the Principles of Palæon tology. Second Edition, revised and greatly enlarged, 2 vols. Svo. With 722 Engravings. 42s. " This book will be found to be one of the best of guides to the -principles of Palæontology and the study of organic remains."— Athenæum. * \ THE ANCIENT LIFE-HISTORY OF THE EARTH. An Outline of the Principles and Leading Facts of Palæontological Science. Crown 8vo, with 270 Engravings. 10s. 6d. " As a text-book of the historical phase of Palæontology it will be indispensable to students, whether specially pursuing geology or biology-"— Quarterly Journal of Science. %■ "Scarcely any recommendation of ours can add to the interest with which all students in natural history will receive the present volume." — Athenæum.

WILLIAM BLACKWOOD & SONS, Edinburgh and London. WORKS ON GEOLOGY AND PHYSICAL GEOGRAPHY. By DAVID PAGE, LL.D., Etc, Professoj of Geology in the Durham University College of Physical Science, Newcastle. INTRODUCTORY TEXT-BOOK OF GEOLOGY. With Engravings on Wood, and Glossarial Index. Twelfth Edition. Re vised and Enlarged by Charles Lapworth, LL.D. j F. G.S., &c.. Professor of Geology and Mineralogy in the Mason Science College, Birmingham. 2s. 6d. " It has not been our good fortune to examine a text-book on science of which we could express an opinion so entirely favourable as we are enabled to do of Mr Page's little work." — Athenæum. ADVANCED TEXT -BOOK OF GEOLOGY. With numerous Engravings, and Glossary of Scientific Terms. Sixth Edition, revised and enlarged. 7s. 6d. THE GEOLOGICAL EXAMINATOR.

A Progressive Series of Questions adapted to the Introductory and Advanced Text- Books of Geology. Tenth Edition. 9d. GEOLOGY FOR GENERAL READERS. A Series of Popular Sketches in Geology and Palæontology. Third Edition, containing several new Chapters. 6s. CHIPS AND CHAPTERS. A Book for Amateur and Young Geologists. 5s. SYNOPSES OF SUBJECTS TAUGHT IN THE GEOlogical Class, College of Physical Science, Newcastle-on-Tyne, Univer sity of Durham. 2s. 6d. INTRODUCTORY TEXT - BOOK OF PHYSICAL GEOGRAPHY. With Sketch - Maps and Illustrations. Twelfth Edition, Enlarged, and Edited by Charles Lapworth, LL.D., F.G.S. , &c, Professor of Geology aud Mineralogy in the Mason Science Col lege, Birmingham. 2s. 6d. ADVANCED TEXT -BOOK OF PHYSICAL GEO- GRAPHY. With Sketch-Maps, Engravings, and Glossary of Terms.

Third Edition. Edited by Professor Lapworth, F.G.S., &c. 5s. EXAMINATIONS ON PHYSICAL GEOGRAPHY. A Progressive Series of Questions adapted to the Introductory and Advanced Text-Books of Physical Geography. Sixth Edition. 9d. WILLIAM BLACKWOOD & SONS, Edinburgh and London. INTRODUCTORY TEXT-BOOK OF GEOLOGY PAGE AND LAPWORTH INTRODUCTORY TEXT-BOOK OF GEOLOGY BY DAVID PAGE, LL.D., F.G.S. PROFESSOR OF GEOLOGY IN THE DURHAM UNIVERSITY COLLEGE OF PHYSICAL SCIENCE, NEWCASTLE-ON-TYNE Author of ' Advanced Text-Book of Geology,' ' Handbook of Geological Terma and Geology," * Past and Present Life of the Globe,' ' Geology for General Readers,' 'Introductory aud Advanced Text-Books of Physical Geography,' &c. REVISED AND IN GREAT PART REWRITTEN BY CHAELES LAPWOETH, LL.D., F.G.S. PROFESSOR OF GEOLOGY AND PHYSIOGRAPHY MASON COLLEGE, BIRMINGHAM TWELFTH ASD ENLARGED EDITION WILLIAM BLACKWOOD AND SONS EDINBURGH AND LONDON MDCCCLXXXVIII The Right of Translation is reserved " The study of the structure of the Earth must tend to enlarge the mind of man, in seeing what is past, and foreseeing what must come to pass in the economy of nature ; and here is a subject in which we find an extensive field for investigation, and for pleasant satisfaction."— Hutton's Theory of the Earth, 1795.

Preface

. The object of this little Treatise is to furnish an elementary outline of the science of Geology. In its preparation the utmost care has been taken to present a simple but accurate view of the subject, to lead the learner from things familiar to facts less obvious, and from a knowledge of facts to the consideration of the laws by which they are governed. By adopting such a method, Geology, instead of being a dry accumulation of facts, and its study a mere task of memory, becomes one of the most attractive departments of Natural Science, and affords one of the finest fields for the exercise of the observing and reflective faculties. The treatise, though initiatory, is arranged on a strictly scientific basis, the Author being convinced that the student's progress is best promoted by commencing at once with the technical treatment of his subject, and making him feel that he is step by step acquiring the power to master for himself the higher and more difficult deductions.

Such a course may require closer attention, and cost him a little more labour at the outset; but it will be found, as he advances, to be the more pleasant as well as the more profitable mode of procedure. Whatever may be the defects of this manual, the Author has endeavoured to write as a geologist — to afford the pupil an accurate outline

PREFACE.

. of the science, should he stop short at this stage of his pro gress, and to present him, should he wish to prosecute the study, with a gradual introduction to a more advanced and comprehensive text-book. Edinburgh, August 1854. Second Edition Third „ Fourth m Fifth m Sixth m Seventh i, Eighth m Ninth m Tenth i. Eleventh ,, published August 1855. ii April 1857. April 1860. ii November 1861. M November 1864. June 1867. ii February 1869. n August 1871. „ _4>*A. 1873. ii October 1877. TWELFTH EDITION. In preparing the present edition of this work, I have been desirous, while retaining the general sequence and mode of treatment of the subject adopted in former editions, to make the book, as far as it goes, a brief epitome of our present knowledge of the science of Geology, as viewed from the most recent standpoint.

I have also naturally been anxious that the book should maintain its reputation as affording a general introduction to the science, — suitable, on the one hand, for beginners and for the higher classes in schools ; and on the other, available for the purposes of systematic students of the elementary branches of the subject in evening classes and colleges. In each section, therefore, I have endeavoured to give, with simplicity and clearness, but with as much fulness as is compatible with the size of the work, an ex planation, description, and illustration of the more important facts and principles upon which the science is based. But the progress of Geology has been so great of late years, that in order to bring all the departments up to date, I have found it necessary to recast or rewrite almost the whole of the work, with the exception of the introductory and con cluding chapters.

Those teachers who have been in the habit of using the PEEPACE TO THE TWELFTH EDITION. book will find that the Physical half of the book has been extended by the addition of fresh chapters on Dynamical Geology, and on the Volcanic and Metamorphic rocks, em bodying the recent discoveries in these departments. In Historical Geology the earlier chapters have been rewritten, and the others brought up to date ; special stress has been laid upon the geographical and biological aspects of each of the geological systems ; and a special chapter has been de voted to the Glacial period. For the benefit of those who use this work in schools, a brief recapitulation is appended to each chapter ; and for the use of systematic students of the science, detailed classifi cations of rocks and rock-formations are inserted in smaller type.

In order to include the new matter, the work has been enlarged by the addition of seventy-five pages of letterp_ress. About fifty fresh illustrations have also been added. Some of these are new. For his kindly permission to use the majority I have to thank my friend, Prof. H. Alleyne Nicholson, M.A., of Aberdeen University. CHAS. LAPWORTH. Mason College, Birmingham, February 1888. CONTENTS. INTRODUCTORY. CHAP. PAGE I. Geology. Objects, Scope, and Practical and Theoretical Bearings of the Science, 9

Part I

— DYNAMICAL GEOLOGY. TREATING OF THE AGENCIES OPERATING ON THE CRUST OF THE GLOBE, AND MODIFYING ITS STRUCTURE AND CONDITIONS. II. Sub-aerial or Exterior Agencies. Atmospheric, Aqueous, and Organic, ..... 19 III. Subterranean or Internal Agencies. Volcanoes, Earthquakes, and Secular Upheaval and Depression, .......... 45

Part II

—PETROLOOICAL GEOLOGY. TREATING OF THE COMPOSITION, STRUCTURE, AND ARRANGE MENT OF ROCKS. IV. The Stratified or Aqueous Rocks. Their Composition, Structure, Classification, and Mode of Arrangement in the Earth's Crust 58 V. The Igneous Eocks. Their Composition, Structure, and Classification, . . 75 VI. The Igneous Eocks (continued). Their Arrangement and Distribution, CONTENTS. VII. The Altered and Metamorphic Eocks. Agencies and Varieties of Alteration and Metamorphism , . .97 VIII. The Metamorphic Eocks (continued). The Foliated and Schistose Eocks : their Characters, Mode of Origin, Classification, &c 106

Part III

— HISTORICAL GEOLOGY. a description of the characteristic lithological AND PALÆONTOLOGICAL FEATURES OF THE SUCCESSIVE BRITISH GEOLOGICAL SYSTEMS. INTEODUCTOEY. IX. Classification of the Stratified Eocks into Formations and Systems, 121 X. Palæontology, or the Science of Fossils, . . . 134

Section I

— EOZOIC PERIOD. XI. Archæan or Pre-Cambrian Eocks, 143

Section II

-PEOTEROZOIC PERIOD. XII. The Cambrian System. Including the Harlech, Menevian, Lingula Flags, and Tremadoc Series, 151 XIII. Ordovician System. Including the Arenig, Llandeilo, and Caradoc or Baia Series, 160 XIV. Silurian System. Including the Llandovery, Wenlock, and Ludlow Series, . 169

Section III

— DEUTEROZOIC PERIOD. XV. Devonian System. Including the Lower, Middle, and Upper Devonian ; and the Lower and Upper Old Red Sandstone, .... 179 XVI. Carboniferous System. Including the Carboniferous Limestone, Millstone Grit, and Coal-Measure Formations, CONTENTS. 7 XVII. Permian or Dyassic System. Including the Magnesian Limestone and Permian Sandstone of Britain ; and the Rothliegende and Zechstein Formations of Germany, 211

Section IV

— MESOZOIC PERIOD. XVIII. Triassic System. Including the Bunter, Keuper, Muschelkalk, and Rhætic Formations of Germany and their British Equivalents, . 219 XIX. Jurassic System. Including the Liassic and Oolite Formations, . . . 230 XX. Cretaceous System. Including the Upper Cretaceous or Chalk ; and the Lower Cretaceous, or Neocomian and Wealden Series, . . 245

Section V

— CAINOZOIC PERIOD. XXI. Tertiary System. Including the Eocene, Oligocene, Miocene, and Pliocene Series, 259

Section VI

- QUATERNARY OE ANTHROPOZOIC PERIOD. XXII. Pleistocene or Glacial Formations, .... 277 XXIII. Recent or Post-Glacial Formations, .... 285 XXIV. Review of the Stratified Systems, .... 304 Glossarial Index, GEOLOGY. CHAPTEK I. INTRODUCTORY OUTLINE. Objects and Scope of the Science. i. Geology (from two Greek words — ge, the earth, and logos, a discourse or reasoning) embraces, in its widest sense, all that can be known of the constitution and history of our globe. Its object is to examine the various materials of which the accessible parts of our planet are composed, to describe their appearance and relative positions, to investigate their nature and modes of formation, to note the changes they have undergone and are still undergoing, and generally to discover the laws which determine their characters aud arrangements. 2.

As a department of natural science, Geology confines itself more especially to a consideration of the mineral or rocky constituents of the earth, and leaves its present surface configuration to Geography, its vegetable life to Botany, its animal life to Zoology, and the elementary constitution of all bodies to the science of Chemistry. As geologists are unable to penetrate beyond a few thousand feet into the solid sub stance of the globe, their labours are necessarily confined to its accessible exterior. By the scientists of former times it was believed that the interior or "nucleus" of the globe was kept in a molten state by excessive heat, and that this liquid portion was surrounded by the solid outer shell or "crust." Adopting this terminology, geologists speak of the " crust of the globe," meaning thereby that outer portion which is acces sible to human investigation, and about whose nature and his tory they can reason with certainty.

A 3. The materials composing this crust are rocks and mine ral matter of various kinds— granite, basalt, slate, sandstone, marble, coal, chalk, clay, and sand — some hard and compact, others soft and incoherent. No matter what may be their degree of consolidation, they are all without exception known to the geologist as "rocks." These rocks do not occur scattered indiscriminately, nor, when found, do they always ap pear in the same relative position. Granite, for example, may exist in one district, marble in another, coal in a third, and chalk in a fourth. Some occur in regular layers (a) or courses, termed strata (from the Latin word stratum, strewn or spread out), and are known as stratified rocks. Others rise up in irregular masses (m), having no lines nor layers of stratification, and are consequently designated unstratified rocks.

Some lie flat, others slope at high angles ; some are bent into arch like ridges, others into basin-shaped hollows ; some occur in Fig. i. — Stratified and Unsira.iifi.ed Rocks. alternating sheets (a), while others burst through these layers, interrupt their continuity, and rise over them in mountain masses (m). It is evident that rocks differing so widely in composition and structure must have been formed under different circumstances, and by different causes. It becomes the task of the geologist to discover those causes, and to indicate the conditions of the regions in which, and of the periods when, such rocks were produced. 4. When we sink a well, for example, and dig through clays, sands, and gravels, and find them succeeding each other in layers, we are instantly reminded of the operations of water, for it is only by the agency of water that accumulations of clay, sand, and gravel are formed at the present day.

Not only are we thus led to inquire as to the origin of the materials through which we dig, but we very naturally seek next to dis cover whether they were originally deposited in river-courses, in lakes, in estuaries, or along the sea-shore. In our investi gation, again, we may perhaps detect shells, bones, and frag ments of plants embedded in the clays and sands ; and thus we have a further clue to the mode of origin of the strata through which we pass, according as the shells and bones are the remains of animals that lived in the fresh-water lakes and rivers, or inhabited the waters of the ocean. Again, in making a railway-cutting, excavating a tunnel, or sinking a coal-pit, we may pass through many different successions of strata — such as clay, sandstone, coal, ironstone, limestone, and the like ; and in such cases we almost invariably find that each succession of strata contains the remains or impressions of different plants and animals.

Such differences can only be accounted for by supposing each set of strata to have been formed under different arrangements of sea and land, as well as under different conditions of climate ; just as at the present day the lakes, estuaries, and seas of different countries are characterised by their own special accumulations, and by the embedded remains of the plants and animals peculiar to these regions. 5. In making such investigations, the geologist is guided by his knowledge of what is now taking place on the surface of the globe — ascribing the origin of such rocks as resemble recent mineral accumulations, formed by existing agencies, to the same agencies acting similarly in the far distant past. Thus, in the present day, we see rivers carrying down sand and mud and gravel, and depositing them in layers, either in lakes, in estu aries, or along the shores and bottom of the ocean.

By this process many lakes and estuaries have, within a comparatively recent period, been filled up and converted into dry land — the layers of sand and mud gradually consolidating and hardening into rocky strata. We see also the tides and waves wasting away the sea-cliffs in more exposed districts, and accumulating expanses of sand arftl salt-marsh in sheltered localities. By these agencies thousands of acres of land have been washed away by the sea, even within the memory of man ; while by the same means new tracts have been formed in districts formerly covered by the tides and waves. Further, we learn that, during earthquake convulsions, and, on a far grander scale, in those slow movements of elevation and depression the effects of which are only discernible after the lapse of ages, large districts of country have sunk beneath the waters of the ocean ; while in other regions the sea-bed has been elevated into dry land.

And, finally, we are all familiar with the fact that, in the neighbourhood of the present shore-line of the continents, volcanic action is visibly affecting the surface of the globe — converting level tracts into mountain-ridges, throwing up new islands from the sea, and casting forth molten lava and other materials, which in time become hard and crystalline rock masses, like the greenstones and basalts of our older hills. 6. As these forces are at present modifying the surface of the globe, and changing the relative positions of sea and land, so in all time past they must have exerted a similar influence. The exterior of this world of ours is, and has ever been, subject to incessant waste and reconstruction — here wasted and worn down by frosts, rains, rivers, waves, winds, and tides, and there built up again by the deposition of the water-borne materials, by the growth of plants and animals, and by the accumulation of volcanic ejections.

Not a foot of the land we now inhabit but has probably been repeatedly under the sea, and, it may be, the present bed of the ocean has formed repeatedly the habitable dry land. No matter how far inland, or at what elevation above the sea, we now find accumulations of sand and gravel — no matter at what depth we discover strata of sand stone or limestone — we feel confident, from their composition and arrangement, that they must have been formed under water, and brought together by the operations of water, just as layers of sand and gravel and mud are accumulated or deposited at the present day. And as crust movements, earthquakes, and volcanoes break up, elevate, and derange the present dry land — here sinking one portion, there tilting up another, and every where producing rents and fissures — so, we rest assured, must the fractures, derangements, and upheavals among the strata of the rocky crust have been brought about by the operation of similar agents in remote and distant epochs. 7.

By the study of existing operations the geologist obtains a clue to the history of the globe ; and his task is rendered much more interesting, and his results more exact and reliable, by an examination of the plants and animals found embedded in the various strata. At present, the hard coverings of shell fish, the skeletons of fishes, and other sea animals are buried in the mud or silt of lakes and estuaries * rivers carry down the remains of land animals, the trunks of trees and other vegetable drift ; and earthquakes submerge plains and islands, with all their vegetable and animal inhabitants. These remains become enveloped in the layers of mud and sand and gravel formed by the waters, and in process of time many become petrified (Lat. petra, a stone, and fio, I become) ; that is, be come converted into stony matter like the shells found in the rocky strata.

AVhen they are exhumed by the geologist, they give evidence of the physical conditions under which they lived — whether aquatic or terrestrial, tropical, temperate, or arctic. It is perfectly clear that, as at present, so in all former time, the remains of plants and animals must have been similarly preserved ; and as one tribe of plants is peculiar to the dry plain, and another to the swampy morass — as one family belongs to a temperate, and another to a tropical region — so, from the character of the plants embedded in the rocks are we enabled to arrive at some knowledge of the conditions under which they flourished. Again, each tribe of animals has its locality assigned it by peculiarities of food, climate, and the like ; each family has its own peculiar structure for running, flying, swimming, plant-eating or flesh-eating, as the case may be ; and by comparing fossil remains of animals (fossil, from Lat. fossus, dug up — applied to all remains of plants and animals embedded in the rocky crust) with existing races, we are enabled to determine many of the past climatic conditions of the world with considerable certainty.

Of course, the more remote the period, or, in other words, the older and deeper the rocks, the more difficult will be this determination ; just as from a study of antiquarian objects it is easier to arrive at a knowledge of the manners and customs of those who imme diately preceded our own times, than of those whose monuments are older than all written record or oral tradition. 8. By examining, noting and comparing, as indicated in the preceding paragraphs, the geologist finds that the strata com posing the earth's crust can be arranged in groups or series ; that one special set or series is always succeeded by, or underlies another set • and that each series contains the remains of plants and animals not to be found in any other series. Having ascertained the existence of such a sequence among the rocky strata, his next task is to determine that sequence in point of time — that is, to determine the older from the newer series of strata ; to ascertain, if possible, the nature of the plants and animals whose remains are embedded in each set; and lastly, to discover the geographical range or horizontal extent of the successive series.

These series he calls formations, as having each been formed or deposited in lakes, estuaries, or seas, during different arrangements of sea and land, and under influences of climate or other external conditions. It is by a knowledge of these formations, and by a careful digest, comparison, and rigid sequential classification of the information they afford him, that the geologist is en abled to arrive at a broad outline of the history of the globe — imperfect, it is true, but still sufficient to show the numer- BEARINGS OF THE SCIENCE. 14 ous changes its surface has undergone, and the varied and wonderful races of plants and animals by which it has been successively inhabited. Thus not only does geology (a) investigate the origin, composition, structure, and arrangements of the rock-masses that constitute the earth's crust ; but it also endeavours (6) to map out the various mutations of sea and land, from the present moment to the earliest time of which we have any traces in the rocky strata, to restore the forms of extinct plants and animals, to indicate their habits, the climate and conditions under which they grew and lived, and even their connection with existing races.

To the former part of our science we apply the term Physical Geology, and to the latter the title of Historical Geology. Theoretical and Practical Bearings of the Science. 9. Such are the objects and scope of Geology, regarded solely from a tluoretical point of view. This aspect of the science is one of comparatively recent development, but one of high and enduring interest. The problems Theoretical Geology proposes to solve are among the most attractive that can engage the ingenuity of man — leading him from his own position and connection with the present aspects of this planet, back through all its former conditions, as far as science is able to penetrate into the dim remoteness of the distant past. As a legitimate cultivator of natural science, the geologist bases his deductions on definite and well-observed facts * collects, arranges, and compares, with honest and scrupulous care ; and proceeds from phenomena that are easily understood and taking place around him, to the explanation of those that are remote and less apparent.

His object is to unfold the history of our globe as revealed in the composition and arrangement of the rocky crust, with all its embedded remains of past life ; and, whether in collecting data among the hills and ravines, by the sea-cliff or in the mine, or in arranging and drawing frOm these data the warranted conclusions, the earnest student will find Geology at once one of the most healthful and exhil arating, as it is one of the most attractive and expanding, of intellectual pursuits. It requires accurate observation, cau tious comparison, and careful deduction at every step; and the powers of observing correctly, of comparing without bias, a*id of deducing in logical sequence, are among the highest attainments of the human intellect. BEARINGS OF THE SCIENCE. 15 10. Nor is the science, in a practical point of view, of less importance to man.

Deriving, as we do, all our metallic and mineral stores from the crust of the earth, it is of the greatest utility to be able to distinguish correctly between mineral substances, to determine in what positions they occur, to in form the miner with certainty where they are to be found, and with what facilities they can be obtained. Again, the engineer, in tunnelling through hills, in cutting canals, excavating harbours, sinking wells, bringing in water-supplies, and the like, must, to do his work securely and with certainty, base in a great measure his calculations on the nature of the rocky materials to be passed through — information he can obtain only through the deductions of Geology. The architect also, in selecting his material, by attending to the formation and texture of the rock, and observing how it has been affected by the weather in cliffs and ravines, may often avoid the needless expense incurred by the unfortunate selection of a wasting and worthless building-stone ; while his knowledge of geological succession will enable him to detect in different localities the material he requires.

The farmer, in like man ner, whose soils are either formed by the disintegration of the subjacent rocks, or are affected by their retentive or absorbent nature, may learn much useful information from the demon strations of the geologist. The study of physical geography, in relation to the migrations and habitats of plants and animals, the acclimatising and cultivation of certain animals and vegetables, and even touching the development and health of man himself, can only attain the character and position of a science, if treated in connection with the fundamental doctrines of Geolog}T. Again, the artist and landscape-gardener may reap substantial benefit from a study of the science as bearing on surface-configuration a,nd character of scenery ; and though such a knowledge, of itself, will make neither artists nor landscape-gardeners, it will often prevent them from com mitting unpardonable outrages on the landscapes of nature.

Indeed, to all who have to deal with minerals and metals some little knowledge of Geology will be of advantage ; while in a country like Britain, whose mechanical, manufacturing, and commercial greatness depends so intimately upon her subterranean treasures, few subjects can be more deserving of enlightened attention. ir. To acquire a knowledge of the science sufficient for the purposes indicated in the preceding paragraph, is not a very difficult task. The objects of geological research are scattered RECAPITULATION. everywhere around us. Not a quarry we visit by the way side, not a railway-cutting through which we are carried, not a mountain-glen up which we climb, nor a sea-cliff under which we wander, but furnishes, when duly observed, important lessons in geology. A hammer to detach specimens and a bag to carry them in, a sketch-book to note unusual appearances, an observing eye, and a pair of willing limbs, are nearly all the young student requires for the field ; and by inspection and comparison of the rocks and fossils in some museum, and by the diligent use of his text-book, he will very shortly be able to proceed in the science as a practical observer.

Let him note every new and strange appearance, handle and preserve every specimen with which he is not familiar — throwing noth ing aside until he has become acquainted with its nature ; and then, besides obtaining additional knowledge and facilitating his progress, he will shortly acquire the invaluable power of prompt and accurate discrimination. RECAPITULATION. 12. In the preceding paragraphs we have endeavoured to explain that the object of Geology is to investigate the structure of the earth, in so far as that structure is accessible to human investigation. Combining all we know of this rocky structure, from the top of the highest mountain to the bottom of the deepest mine, we find that the portion open to our in vestigation forms but an insignificant film of the four thousand miles which lie between the surface and centre of the globe.