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Show 1G8 ISLAND LIFE. [PART I. h 11 h'ch cannot be will observe everywhere mounds and o ows w I k Iu re-accounted for by the present agencies at wor · . gard to the general surface of the country the pr.esent agencies may be said to be just beginning to carve a new lm.e of features out of the old glacia11y-formed surface. But .so httle progress has yet been made, that the kames, gravel-~ou~d.s, knolls .?~ boulder clay, &c., still reta,in in most cases tbeu ongmal form. The facts here seem a little inconsistent, and we must suppose that Dr. Croll bas somewhat exaggerated the universality and complete preservation of the glaciated surface. Th~ ~mount of average denudation, however, is not a ma:ter of opmwn but of measurement· and its consequences can m no way be evaded. They are, mo;eover, strictly proportionate to the time elap~ed; and if so much of the old surface of the country has certamly been remodelled or carried into the sea since the last glacial epoch, it becomes evident that any surface-phenomena produced by still earlier glacial epochs must have long since entirely disappeared. Rise of the Sea-level connected with Glacial Epochs, a ca~tse of f~trther Denudation.-Tbere is also another powerful agent that must have assisted in the destruction of any such surface deposits or markings. During the last glacial epoch itself th ere were several oscillations of the land, one at least of considerable extent, during which shell-bearing gravels were deposited on the flanks of the Welsh and Irish mountains, now 1,300 feet above sealev- el; and there is reason to believe that other subsidences of the same area, though perhaps of less extent, may have occurred at various times during the Tertiary period. Many writers, as we have seen, connect this subsidence with the glacial period itself, the unequal amount of ice at the two poles causing the centre of gravity of the earth to be displaced, w ben, of course, the surface of the ocean will conform to it and appear to rise in the one hemisphere and sink in the other. If this is the ca~e, subsidences of the land are natural concomitants of a glacial period, and will powerfully aid in removing all evidence of its occurrence. We have seen reason to believe, however, that during the height of the glacial epoch the extreme cold persisted through 1 Climate and Time in their Geological Relations, p. 341. CHAP. IX.] ANCIENT GLACIAL EPOCHS. 169 the SUCC6 '"·' S l.Ve ph ases of precessw· u, an d I· f so, both polar areas would ~robably be glaciated at once. This would cause the abstract.w n of 'a ]a rge quanti· ty of water from the ocean, and a Proportwna. te el evatw· n of the land, · which would react on the accumulatwn of snow and ice, and thus add another to that wonderful serie:3 of physical agents which act and react on each other so as to intensify glacial epochs. But ~bother or not these causes would produce any important fluctuatwns of the .sea-~evel is of comparatively little importance to our present mqmry, because the wide extent of marine Tertiary deposits in the northern hemisphere and their occurrence at considerable elevations above the present sea-level, afford the most conclusive proofs that great changes of sea and land have occurred throughout the entire Tertiary period; and these repeated submergences and emergences of the land combined with sub-aerial and marine denudation, would undoubtedly destroy all those superficial evidences of ice-action on which we mainly depend for proofs of the occurrence of the last glacial epoch. TYhat evidence of ea?"ly Glacial Epochs may be expected.Although we may admit the force of the preceding argument as to the extreme improbability of our finding any clear evidence of th~ superficial action of ice during remote glacial epochs, there IS nevertheless one kind of evidence that we ourrht to find because it is both wide-spread and practically indestr:ctible. ' One of the most constant of all the phenomena of a crlaciated . b country IS the abundance of icebergs produced by the breaking off of the ends of glaciers which terminate in arms of the sea or of the terminal face of the ice-sheet which passes beyond ~he land into the ocean. In both these cases abundance of rocks and debris, such as form the terminal moraines of glaciers on land, are carried out to sea and deposited over the sea-bottom of the area occupied by icebergs. In the case of an ice· sheet it is almost certain that much of the ground-moraine, consisting of mud and imbedded stones, similar to that which forms the'' till" when deposited on land, will be carried out to sea with the ice and form a deposit of marine "till" near the shore. It has indeed been objected that when an ice-sheet covered |