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Show 112 ISLAND LTFE. [PAin' I. when the ice was at its maximum, it extended not only over the land, but far out to sea, covering all the Scottish islands, and stretching in one connected sheet to Ireland and W, ales, where all the evidences .of glaciation are as well marked as m Scotland, thourrh the ice did not of course attain quite so great a thickness.1 It is evident that the change of climate requisite to produce snch marvellous effects in the British Isles could not have been local, and we accordingly find strikingly similar proofs that Scandinavia and all northern Europe have also been covered with a huge ice-sheet; while we have already seen that a similar gigantic glacier buried the Alps, carrying granitic blocks to the Jura, ·where it deposited them at a height of 3,450 feet above the sea; while to the south, in the plains of Italy, the terminal moraines left by the retreating glaciers have formed extensive hills, those of Ivrea, the work of the great glacier from the Val d' Aosta being fifteen miles across, ancl from 700 to 1,500 feet high. Glacial Phenomena in North Amerwa.-In North America the marks of glaciation are even more extensive and striking these openings in the valleys that the ''till" is said to be found, and ~lso ~n the lowlands where an ice-sheet must have extended for many m1les m every direction. In these lowland Yalleys the "till" is both thickest and most wide-spread, and this is what we might expect. At first, when tlw glaciers from the mountains pushed out into these valleys, ~bey would grind out the surface beneath them into hollows, and the dramage-water would carry away the debris. But when they spread all over the surface from sea to sea, and there was little or no drainage water compared to the enormous area covered with ice, the great bulk of the debris must gather under the ice wherever the pressure was least, and the ice would necessarily rise as it accumulated. Some of the mud would no doubt be forced out along lines of least resistance to the sea, but the friction of the stone-charged "till" would be so enormous tha~ it would be impossible for any large part of it to be disposed of in this way. . 1 That the ice-sheet was continuous from Scotland to Ireland IS proved by the glacial phenomena in the Isle of Man, where "till" similar to that in Scotland abounds and rocks are found in it which must have come from Cumberland and Scotland, as well as from the north of Ireland. This would show that glaciers from each of these districts reached the Isle of Man, where they met and flowed southwards down the Irish Sea. ~comarks are traced over the tops of the mountains which are nearly 2,000 :f eet high. '(See A Slcetch of the Geology of the Isle of Ma.n, by John Horno, F.G.S. Tmns. of the Edin. Geol. Soc. Vol. II. pt. 3, 1874.) C'IJAP. VII.] THE GLACIAL EPOCH. 113 than in Europe, stretching over the whole of Canada and to the south of the great lakes as far as latitude 39°. There is, in all these countries, a wide-spread deposit like the "till" of Scotland, produced by the grinding of the great ice-sheet when it was at its maximum thickness ; and also extensive beds of moraine-matter, true moraines, and travelled blocks, left by the glaciers as they retreated towards the mountains and finally withdrew into the upland valleys. There are, also, both in Britain, Scandinavia, and North America, proofs of the submersion of the land beneath the sea to a depth of upwards of a thousand feet; but this is a subject we need not here enter upon, as our special object is to show the reality and amount of that wonderful and comparatively recent change of climate termed the glacial epoch. Many persons, even among scientific men, who have not given much attention to the question, look upon the whole subject of the g1acial epoch as a geological theory made to explain certain phenomena which are otherwise a puzzle; and they would not be much surprised if they were some day told that it was all a delusion, and that Mr. So-and-so had explained the whole thing in a much more simple way. It is to prevent my readers being imposed upon by any such statements or doubts, that I have given this very brief and imperfect outline of the nature, extent, and completeness of the evidence on which the existence of the glacial epoch depends. There is perhaps no great conclusion in any science which rests upon a surer foundation than this ; and if we are to be guided by our reason at all in deducing the unknown from the known, the past from the present, we cannot refuse our assent to the reality of the glacial epoch of the northern hemisphere in all its more important features. Effects of the Glacial Epoch on Animal Life: Warm and Cold Periods.-It is hardly necessary to point out what an important effect this great climatal cycle must have had upon all living things. When an icy mantle crept gradually over much of the northern hemisphere till large portions of Europe and North America were reduced to the condition of Greenland now, the ' greater part of the animal life must have been driven southward causing a struggle for existence which must have led to the ~xtermination of many forms, and the migration of others I |