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special study. The nature, characters, and distribution of the till (boulder-clay) and erratics, striated rock-surfaces, rockbasins, kaims and escars, beds of arctic shells, and the local alternations of sands, gravels, clays, and lignites with the till, bave readily attracted the attention of geologists, and the consideration of their probable origin fully occupies the philosophic mind and feeds the scientific imagination. As similar natural features abound in other parts of the world, scientists of many sorts, having concluded that ice must have been the chief agent concerned with these phenomena, have carefully observed its various physical conditions, and have invented many hypotheses about its movements and their results. The compilation of these facts and opinions, here offered by the Author, is naturally affected by his own predilections and conscientious beliefs, which occasionally lead him to oppose or ignore some even late researches on icefields and superficial drifts.
The "List of Authors quoted or referred to" (about 580) at pages 832-838 indicates the wide extent of the literature to be dealt with; and Prof. J. Geikie intimates with truth that the list does not exhaust what has been published about the facts and inferences relating to glacial phenomena and the former existence of wide-spread and intense action of Ice on some parts of the Earth's surface.
In the first (1874) and second (1878) editions of this book, "The Great Ice-Age and its relation to the Antiquity of Man," Scotland supplied, as now, the typical facts and phenomena of a Glacial region, with its subsequent (Post-glacial) conditions; and, as great changes of climate seemed to be involved in that geological history, these were considered on the basis then regarded as the best available. Clearer recognition of leading facts, and their more perfect explanation, have much modified the hypotheses that were then received as good and useful; and the Author in this, the Third Edition, endeavours to give his opinion clearly on the results both of the new and extended examinations of the physical phenomena, and of many criticisms on the one haud, and helpful suggestions on the other.
In the order of the subject-matters of the book, the glacial deposits seen in Scotland come first, with the boulder-clay, its composition, relative position, special characters, and local features. Various modes of its origin, hypothetically advanced, whether by violent aqueous agency, or by the mediation of glaciers and icebergs, are dealt with; and a careful account is given of glaciers, and of the modes and results of their action on the surface of the ground.
Chapter IV. is devoted to icy Greenland and its supposed constitution of separate islands; and treats of Arctic glaciers, with icebergs, foot-ice, and sea-ice, and the littoral accumulations of earthy and stony débris. These are illustrative of much of the ice-work which has modified the surface of Scotland.
The glacial origin of the Till being accepted, there follows a
consideration of the probable modes and processes of ice-action in producing its clay and stones, and in grinding, rounding, polishing, and scoring the latter (Chapters V. and VI.). Doubtless the glacier, as is well known, could do all this; but it is very doubtful indeed if the boulder-clay was a persistent floor of any glacier; or that the stones were not all derived from the frozen rocks of the valley giving off their débris from the sides to its surface. No ice could tear solid blocks from the floor. Water running beneath the glacier carries along abundant mud and sand amidst the grinding pebbles of the so-called "moraine profonde;" and, though some layers are lodged in hollows and quiet recesses, the rest passes on towards the glacier's foot, wherever that may be, in river, fiord, or open sea. These considerations, as to whether it was a persistent bed on the glacier's floor (quite impossible), or accumulated at its foot by water, with or without icebergs and ice-floes, necessarily affect the explanation of the local occurrences and disposition of the Boulder-clay, also of its recurrence after the existence of a land-surface, and of its occasional fossil shells and other organisms.
The physical evidences of ancient ice-sheets ("mers de glace") affecting Scandinavia and the British Isles, and regarded as having been more or less conterminous or continuous, are enlarged upon, and Pl. IX., opposite page 437, delineates the supposed greatest extent of this glaciation. Whether, however, this condition existed over the European area uniformly at one and the same time, and how thick the main ice-sheet may have been, are points not yet proved to the satisfaction of all. In this, as in some other cases, the Author prefers to collate and use the ideas and statements supporting his views, without reference to contrary opinions.
Stratified and unstratified beds associated with the Scottish Till form the text of Chapters VII. to XIII. At page 89 allusion is made to "gravel, sand, silt, mud, brick-clay, and peat" that underlie the lower till, and to similar beds lying upon it and covered by the upper till. Some such deposits contain freshwater fossils, and in some others there are marine shells. These "beds below and in the till" are stated (p. 192) to indicate pauses in its formation (though the lowest, being beneath, would seem to have been formed previously!), and to have been due to ordinary water-action. They are often of great interest, and have been carefully examined by the geologists of Scotland wherever met with in exposures, diggings, and borings. From them much has been gathered as to the probable changes of fauna and flora under different successive climates (" recurrent cold and warm periods "); also of the former varying lines of rivers and lakes, and of the conditions and features of the country during the formation of these "glacial" and "interglacial" deposits.
Chapters XIV. to XVII. treat of glacial deposits overlying the till of Scotland, and their origin. These are the åsar (escars) and kames, ridges, mounds, and terraces of gravel and sand, due to
the material of moraines of probably decaying glaciers having been shifted and arranged by strong streams of water running therefrom; hence they are said to have been formed by fluvio-glacial agency. Scattered blocks (" erratic "), brought from a distance and left on the melting of the ice, and morainic accumulations left in mountain-valleys, come in the same category as the above.
Lakes and rock-basins occupy Chapters XVIII. and XIX. Several modes of origin are known for lakes, and some are briefly mentioned here. For lakes in the supposed ice-ploughed hollows in solid rock, a favourite subject with his friend the late Sir Andrew Ramsay, Prof. J. Geikie advances all the evidence he can, ignoring what has been brought forward and published to the contrary.
Chapters XX. and XXI. explain that other superficial deposits later in age than the "upper till" were brought about by the existence of "district ice-sheets and local glaciers," belonging to Dr. J. Geikie's "third glacial epoch" (p. 283). These, he thinks, succeeded "interglacial" conditions, and were locally coincident with some shelly deposits on the sea-board, especially along the Firths of the Clyde and Forth. It is to this period that the Author now refers the origin of the "parallel roads of Glenroy" by the intervention of the great western ice crossing the neighbouring valleys lower down and thereby causing lakes, as first suggested by Agassiz, and subsequently defined by Jamieson and Prestwich (p. 283).
Later formations, either within, or just subsequent to the Glacial Period are noted in Chapters XXII. & XXIII., as peatbogs, raised beaches, sand dunes, river-terraces, and old lake-beds, bearing recognizable evidence of their relative age, contemporary with a partial submergence of the land; and with a cold climate, destructive of the forests and giving rise to some highland glaciers. A repetition of the foregoing or similar conditions; and then the last elevation of the land, and the arrival of the present state of things, constitute part of the eleven successive changes, from the "lower till" to the existing land and sea, succinctly enumerated at page 325, so far as Scotland is concerned.
We are now led to the consideration of glacial phenomena met with (1) in England and Ireland; (2) Northern Europe; (3) Central Europe; (4) the Alps, and other parts of Europe, in Chapters XXIV. to XXXVI. This last chapter enumerating five glacial epochs, each with its interglacial epoch, brings us to present or, rather, prehistoric times, and comprises long periods during which Man has existed in this part of the World.
The last or :6 present" epoch (p. 612) was "marked in Britain by the retreat of the sea to its present level, and by the return of milder and drier conditions, and the final disappearance of permanent snow-fields."
The Author arranges the "glacial succession" in the British Islands as follows (beginning with the lowest):
1. Weybourn crag and Chillesford clay. Marine deposits with pronounced arctic fauna.
2. Forest-bed of Cromer. Temperate flora; Elephas meridionalis, E. antiquus, Rhinoceros Etruscus, Hippopotamus, &c. 3. Lower Boulder-clays and associated fluvio-glacial deposits. Ground-moraines (? )*, &c., of most extensive ice-sheet. 4. Marine, freshwater, and terrestrial accumulations; basin of Moray Firth; basin of Irish Sea; Lanarkshire; Ayrshire; Edinburghshire, &c.; Hessle gravels; Sussex beach-deposits, &c.; Settle Cave, &c.
5. Upper Boulder-clay and associated fluvio-glacial deposits. Ground-moraine (?) of ice-sheet which extended south to the Midlands of England.
6. Freshwater alluvia underlying oldest peat-bogs; probably a considerable proportion of our so-called "post-glacial " alluvia. Temperate flora and fauna; Irish deer, red deer, Bos primigenius, &c.
7. Boulder-clays and terminal moraines of mountain-regions. The 100-feet beach of Scotland; arctic-plant beds. Moraine accumulations of district- and large valley-glaciers; arctic marine fauna; snow-line at 1000 to 1600 feet; arctic flora. 8. Lower buried forest. Temperate flora and fauna. 9. Peat overlying the "lower buried forest;" Carse-clay and raised beaches; valley-moraines; corrie-moraines. 10. Upper buried forest. Temperate flora and fauna. 11. Peat overlying upper buried forest; low-level raised beaches; high-level valley- and corrie-moraines. glaciers in the most elevated regions; snow-line at 3500 feet.
The contemporary European formations for each of the eleven above-mentioned epochs follow at pages 614 and 615.
In arriving at his conclusions as to the succession of the "Glacial" deposits, and as to the conditions under which they were probably formed, the Author has evidently been specially influenced by his belief that the main agents have been enormous "icesheets" and their supposititious "ground moraines," of great thickness and consistency; and not separate glaciers, icebergs, ice-floes, and coast-ice, making and distributing mud and sand with gravel and boulders, and helping to make up the full score of striæ. For the changes of climate accounting for the coming and going of the seemingly probable arctic and temperate conditions, indicated by the glacial and non-glacial phenomena, the Author favours the astronomical (Croll and Ball) hypothesis.
Prof. Geikie argues assiduously and plausibly on all these points, without allowing full weight to published objections. An ideal moraine profonde, such as would suit a widely continuous and thick "boulder-clay," would far exceed the limited dimensions
See above, p. 381, for some remarks on the moraines profondes, supposititious as far as the great boulder-clays are concerned.
and conditions of the movable mixture of mud, sand, and stones, that grind and are ground on a glacier's floor, until they find exit at the terminal moraine, or are taken off by icebergs to form submarine till. If the maxima, or even the existence, of icesheet periods are to be calculated according to the extent and thickness of such supposed boulder-clay floors (hundreds of feet thick), the calculations must be erroneous: whilst doubtless the distribution, by river, lake, and sea, of the clay and stones resulting from the grinding work of the glaciers simply and satisfactorily meets many of the geological requirements.
A very large amount of instructive geological detail, obtained by personal research and from a variety of sources, is arranged systematically in treating of stratified and other formations whose origin was in any way associated with periods of extreme cold and with milder intermediate times,-for Scotland (as we have seen) in Chapters II. to XXIII.; for England in Chapters XXIV. to XXVII.; and for Ireland in Chapter XXVIII. The amount of information collected for Europe (Northern, Middle, Alpine, and Western) in Chapters XXIX. to XXXVI. is extensive, and is carefully arranged; resulting (pages 607-615) in the Author's determination of six glacial epochs, each with its sequent interglacial period; the last of the latter exists at present time.
He supports this frequency of climatal changes by reference to his interpretation of successive glacial and non-glacial deposits, and the concomitant topographical modifications in the cutlines, heights, and depths of land and water. These being more numerous and complicated than they were thought to be when his earlier editions were published.
A careful study of the many published accounts of the nature, extent, and origin of the Loess or Lehm is given in Chapter XXXVIII. This Pleistocene deposit is described as a fluvioglacial inundation-deposit, largely modified and rearranged by wind, snow, and dust-storms, and naturally containing multitudinous mammalian bones, many land-shells, and other rarer fossil remains. It necessarily constitutes an important link among the results of the dying ice-age, and the gradual disappearance of the glaciers.
The advent of a cold climate was indicated for Europe in the Pliocene times; and the maximum of cold occurred in the “Second Glacial Epoch." Then the alternating cold and temperate periods successively declined in importance. Palæolithic Man is regarded (p. 689) as having appeared in Europe in Pleistocene times, probably not before the "Second Interglacial Epoch " (Elephasantiquus stage); and, surviving the "Third Glacial Epoch," he probably retired to Southern France, and to Switzerland and the country of the Danube.
Respecting the relative chronology of the Glacial Epoch and the appearance of European Man, it may be well to note that Dr. Croll at first suggested that the beginning of the Glacial Period may