168 



ISLAND LIFE. 



[part I. 



will observe everywhere mounds and hollows which cannot be 

 accounted for by the present agencies at work. ... In re- 

 gard to the general surface of the country the present agencies 

 may be said to be just beginning to carve a new line of features 

 out of the old glacially-formed surface. But so little progress 

 has yet been made, that the kames, gravel-mounds, knolls of 

 boulder clay, &c., still retain in most cases their original form." ^ 

 The facts here seem a little inconsistent, and we must suppose 

 that Dr. Croll has somewhat exaggerated the universality and 

 complete preservation of the glaciated surface. The amount of 

 average denudation, however, is not a matter of opinion but of 

 measurement ; and its consequences can in no way be evaded. 

 They are, moreover, strictly proportionate to the time elapsed ; 

 and if so much of the old surface of the country has certainly 

 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 Qiiust have long since entirely 

 disappeared. 



Rise of the Sea-level connected with Glacial Ejpochs^ a cause of 

 further Denudation. — There 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 there 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 sea- 

 level ; 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, when, 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 case, 

 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 

 ^ Climate and Time in their Geological Relations, p. 341. 



