XXVI.] LAKES AND INLAND SEAS. 279 5. (2) A further feature in the distribution of lakes, which may be observed on maps, is the more or less abundant occurrence of these sheets of water among FIG. 53.-Section of a lake-basin lying in a hollow of superficial detritus. mountains. Take Europe as an illustration. Even in the comparatively low mountain groups of Scotland, Cumberland, and Wales, lakes abound, forming one of the great charms of the well-known scenery of these districts. Among the Alps a series of large lakes occurs on each side of the main axis of the chain, and innumerable minor sheets of water occur scattered at all heights among the central mountains, up even to the borders of the snow-line. All mountain systems, indeed, have not FIG. 54.-Section of a lake dammed up by a barrier of earth or gravel. the same abundance of water-filled hollows; in some there are few or none. Lakes among mountains may be, in some cases, hollows formed during the elevation of the mountains (Lesson XXIX.); in other examples, like those referred to in Art. 4, they may either have had their basins scooped out by glaciers or formed by the irregular piling up of ice-borne debris (Fig. 54). In most volcanic districts, lakes occur in cavities which e been formerly blown open by explosions from 1892, Augu FIG. 163.-The Planet Mar 1892. The South Polar Now Ready. Globe 8vo. Price 2s. 6d. Geology for Beginners BY W. W. WATTS, M.A., F.G.S. FORMERLY LECTURER TO THE CAMBRIDGE UNIVERSITY EXTENSION NOW ASSISTANT-PROFESSOR IN GEOLOGY AT THE WITH THree hunDRED AND TEN ILLUSTRATIONS London MACMILLAN AND CO., LIMITED NEW YORK: THE MACMILLAN COMPANY All rights reserved Ar the present time there appears to be needed a small book on Geology which, while short and elementary in treatment, is accurate and fairly up to date. Further, it is well that sections and diagrams should be supplemented by photographs of hand-specimens and microscopic slides of rocks, and of the natural exposures where rocks are to be seen in the field. The author has kept both these aims in view while planning his work on the lines of the revised syllabus of the Science and Art Department. While primarily intended for use in preparation for the elementary stage of the Science and Art Examination, care has been taken to make the book suitable for school work and for the examinations of the Oxford and Cambridge Schools' Examination Board. With this object in view there have been placed at the end of the chapters all the Questions set in the Science and Art Examination during the last twenty years, and those of the Oxford and Cambridge Board for ten years. Naturally in writing this book no other elementary work of the class has been consulted, but no one who has ever read the late Professor Green's classic work on Physical Geology can help being attracted by the lucidity of his style and influenced by the charm of his methods. The writer owes an especial debt for the advice and assistance given him when acting as Professor Green's deputy on two occasions several years ago. The principal methods pursued in two or three of the earlier chapters, and the general practical aim of the book as a whole, are due directly to Professor Green's advice. CONTENTS CHAPTER I. Introduction; II. Study of a Piece of Stone at Home; III. Study of Rocks out of Doors; IV. Wear and Tear of Rocks by the Weather and Springs; V. Denudation by Rivers and Glaciers; VI. Marine Denudation-Rate of Work; VII. Rock-building by Sediments; VIII. Rock-structures and Earth-movement; IX. Faulting, Cleavage, and Joints; X. Minerals; XI. Sedimentary Rocks; XII. Volcanoes; XIII. Volcanic Rocks; XIV. Plutonic Rocks; XV. Foliated Rocks; XVI. Fossils; XVII. Principles of Historical Geology; XVIII. The Eozoic and Older Palæozoic Groups; XIX. The Paleozoic Group -Deutozoic Division; XX. The Neozoic Group-Mesozoic Division; XXI. The Neozoic Group-Cainozoic Division; XXII. The Origin of Landscape; XXIII. Economic Geology. 76 ROCK-BUILDING BY SEDIMENTS CHAP. brought by rivers and currents. The pebbles are laid down as shown in Fig. 49, in such a way as to oppose the greatest resistance to being lifted again by currents. Direction of Current FIG. 49.-Deposition of pebbles under the influence of a current. The sand is interesting because the perpetual movement of the waves imprints on it the beautiful ribbed surface known as ripple-mark, see Fig. 50. Shell-fish, crustaceans like crabs FIG. 50.-Ripple-marking on sand; also formation of sea-caves by denudation along planes of bedding and jointing. (From a photograph by Mr. G. T. Atchison: copyright.) and lobsters, worms, and other animals, moving over it between tide-marks, leave their tracks and burrows, which may occasionally be preserved if a coating of mud or fine sand is deposited upon them. Even rain-prints indented by a passing shower, and cracks caused by the drying and shrinking of sand or mud under the sun's rays where it is left dry by the tide, may under favourable circumstances be preserved (Fig. MACMILLAN AND CO., LIMITED NEW YORK: THE MACMILLAN COMPANY Academy "It is lucid and attractive in a degree without precedent among geological handbooks. Facts which in most other books of the kind are presented in isolation are here given in a chain of interesting relations, by means of which they are impressed indelibly on the learner's memory. One feature of the book which will render it specially suitable to young students is the large space given to certain details which are usually delegated to mineralogy. The woodcuts (most of which, it is stated, have been drawn expressly for this book) are extremely good." Educational News-" As this is the latest manual of Geology it is also without question the best. . . ." Saturday Review" Clear in expression and lucid in method, the work exhibits all the literary grace for which Dr. Geikie is so highly distinguished." |