THE '" GREAT ICE A6E." 125 



over which the glacier would slide without either grinding 

 it lower by erosion or raising it higher by deposition. 



But what must be the nature of this deposit ? It is evi- 

 dent that it cannot be a mere moraine consisting only of the 

 larger fragments of rock such as are now deposited at the 

 foot of glaciers that die out before reaching the sea. Nei- 

 ther can it correspond to the glacial silt which is washed 

 away and separated from these larger fragments by glacial 

 streams, and deposited at the outspreadings of glacier tor- 

 rents and rivers. It will correspond to neither the assorted 

 gravel, sand, nor mud of these alluvial deposits, but must 

 be an agglomeration of all the infusible solid matter the 

 gacier is capable of carrying. 



It must contain, in heterogeneous admixture, the great 

 boulders, the lesser rock fragments, the gravel chips, the 

 sand, and the slimy mud; these settling down quietly in 

 the cold, gloomy waters, overshadowed by the great ice- 

 sheet, must form just such an agglomeration as we find in 

 the boulder clay and tills, and lie just in those places where 

 these deposits abound, provided the relative level of land 

 and sea during the glacial epoch were suitable. 



I should make one additional remark relative to the 

 composition of this deposit, viz., that under the conditions 

 supposed, the original material detached from the rocks 

 around the upper portions of the glaciers would suffer a far 

 greater degree of attrition at the glacier bottom than it ob- 

 tains in modern Alpine glaciers, inasmuch as in these it is 

 removed by the glacier torrent when it has attained a certain 

 degree of fineness, while in the greater glaciers of the 

 glacial epoch it would be carried much further in associa- 

 tion with the solid ice, and be subjected to more grinding 

 and regrinding against the bottom. Hence a larger pro- 

 portion of slimy inud would be formed, capable of finally 

 induring into stiff clay such as forms the matrix of the till 

 and boulder clay. 



The long journey of the bottom debris stratum of the 

 glacier, and its final deposition when in a state of neutral 

 equilibrium between its own tendency to repose and the 

 forward thrust of the glacier, would obviously tend to ar- 

 range the larger fragments of rock in the manner in which 



