FORMER CONDITIONS OF THE SURFACE. 
529 
sudden successive changes in the relations of land and water, these glaciers were 
broken up, and fragments of them, constituting isles with included blocks, were 
transported during long periods to the south. As it is demonstrable, that the whole 
region has undergone great variation of relative level subsequent to this dispersion 
of the blocks, by the conversion of the ancient bed of the sea into a continent, so 
are we disposed to think, that this change was caused by general expansive forces 
from beneath, which, unable to obtain a vent through any fissures, in the uniform 
crust of sediment which is spread over this undisturbed region, raised up en masse 
the strata by which they were repressed. 
Under the belief that the sea covered all these flat regions when the detritus was 
dispersed, it is not difficult to explain, why the larger blocks are more frequently- 
found associated with clay than sand. Icebergs such as now float in the Antarctic 
and Pacific Seas, and to which we have elsewhere referred 1 * * * , must we conceive have 
offered rough, jagged and unequal bottoms, which impinging on submarine undu- 
lations of mud or clay, would naturally be thereby arrested and often held in oscil- 
lation until the glacial masses dissolved and deposited their loads ; but the loose 
and incoherent sands, often arranged over widely-extended flat surfaces, would op- 
pose comparatively no such obstacles, and the hard, icy mass easily forcing onwards 
would proceed until its bottom impinged upon a bank. We shall presently ad- 
vert to Swedish examples which, illustrating this process, serve to explain, why 
the greater number of the large boulders are found in groups on the summits and 
slopes of hills. In the meantime, if such were the ancient conditions under which 
the boulders were deposited over the plains of Russia and Northern Germany, it is 
clear that we cannot deny 7 ' the application of this aqueous portion ol the glacial 
theory to those tracts We will not here recapitulate our general reasoning foi the 
adoption of such views, which we formerly applied to Scotland ; not from any study 
of Alpine phenomena, but from the facts then before us, and the perusal ot the 
persuasive writings of Mr. C. Darwin. We then thought, that under certain changes 
of sea and land, as pointed out by him, ice might have been formed on the shores 
of former islands which now appear as the mountains of Scotland and Cumberland. 
Though we now see reasons to limit and modify this view in reference to those 
1 See Silurian System, p. 541, et ante , in which some of the icebergs of the Pacific are described as 
being 300 feet above the’sea, and as having a depth of near 2000 feet below its surface. The practical 
application of this phenomenon in illustrating the condition of certain Swedish phenomena w.ll be given 
towards the close of this chapter. 
