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Show 108 ISLAND LIFE. [PART I. head of the Lake of Geneva, since it spread over the whole of the great valley of Switzerland, extending from Geneva to N eufchatel, Berne, and Soleure, and even on the flanks of the Jura, reached a maximum height of 2,015 feet above the valley. The numerous blocks scattered over the Jura for a distance of about a hundred miles vary considerably in the material of which they are composed, but they are found to be each traceable to a part of the Alps corresponding to their position, on the theory that they have been brought by a glacier spreading out from the Rhone valley. Thus, all the blocks situated to the east of a central point G (see map) can be traced to the eastern side of the Rhone valley (l e d), while those found towards Geneva have all come from the west side (p h). It is also very suggestive that the highest blocks on the Jura at G have come from the eastern shoulder of Mont Blanc in the direct line h B F G. Here the glacier would naturally preserve its greatest thickness, while as it spread out eastward and wcstwanl it would become thinner. We accordingly finn that the travelled blocks on either side of the central point become lower and lower, till near Soleure and Geneva they are not more than 500 feet above the valley. The evidence is altogether so conclusive that, after personal examination of the district in company with eminent Swiss geologists, Sir Charles Lyell gave np the view he had first adopted-that the blocks had been carried by ice during a period of submergence-as altogether untenable.1 The phenomena now described demonstrate a change of climate sufficient to cover all our higher mountains with perpetual snow, and fill the adjacent valleys with huge glaciers at least as extensive as those now found in Switzerland. But there are other phenomena, best developed ·in the northern part of our islands, which show that even this state of things was but the concluding phase of the glacial period, which, during its maximum development, must have reduced the northern half of our island to a condition only to be paralleled now in Greenland and the Antarctic regions. As few persons besides professed geologists are acquainted with the weight of evidence 1 Antiquity of Man, 4th Ed. pp. 3 !0-348. CTIAI'. VII.) THE GLACIAL EPOCH. 10!) for this statement, and as it is most important for our purpose to understand the amount of the climatal chanrres the northern hemisphere has undergone, I will endeavou; to make the evidence intelligible, referring my readers for full details to Dr. James Geikie's descriptions and illustrations.I Glacial Deposits of Seotland: the " Till."-Over almost all the lowlands and in most of the highland valleys of Scotland there. arc i~monse superficial deposits of clay, sand, gravel, o~ ~nft, whiCh can be traced more or less directly to glacial actiOn. Some of these are moraine matter, others are lacustrine deposits, while others again have been formed or modified by the sea during periods of submergence. But below them all, and often resting directly on the rock-surface, there arc extensive layers of a very tough clayey deposit known as "till." The till is very fino in texture, very tenacious, and often of a rock-like hardness. It is always full of stones, all of which arc of rude for.m, but with the angles rubbed off, and almost always covered w1th scratches and strice often crossincr each other in various directions. Sometimes the stones are s~ numerous that there seems to be only just enough clay to unite them into a solid mass,. and. they are of all sizes, from mere grit up to rocks m~ny f~et .m d1ameter. Tho "till" is found chiefly in the lowlymg d1stncts, where it covers extensive areas sometimes to a depth of a hundred feet; while in the highlands it occurs in much smaller patches, but in some of the broader valleys forms terraces which have been cut through by the streams: Occasionally it is found as high as two thousand feet above the sea, in hollows or hill-sides, where it seems to have been protected from denudation. The "till'' is totally unstratified, and tho rock-surfaces on which it almost always rests are invariably worn smooth, and mnch grooved and striated when the rock is hard; but when it is soft or jointed, it frequently shows a greatly broken surface. Its colour and texture, and the nature of tho stones it contains all correspond to the character of the rock of tho district wher~ it occurs, so that it is clearly a local formation. It is often 1 . ~he Gl'eat Ice Age and its R elation to tl1e Antiquity of Man. By James Ge1k!C, F.H.S. (I::;bislcr and Co., 1874.) |