168 
ISLAND LIFE. 
[part I. 
will observe everywhere mounds and hollows which cannot he 
accounted for hy the present agencies at work. ... In re- 
gard to the general surface of the country the present agencies 
may he said to he 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.” 1 
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 hut 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 must have long since entirely 
disappeared. 
Rise of the Sea-level connected with Glacial Epochs, 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, «f 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 
1 Climate and Time in their Geological Relations, p. 341. 
