168 SCIENCE IN SHORT CHAPTERS. 



the currents that would necessarily be formed by the inter- 

 change of water of varying specific gravities. Deposition 

 would thus take place in this deeper water, continually shallow- 

 ing it or bringing up the sea-bottom nearer to the ice-bottom. 



This raising of the sea-bottom must occur not only here, but 

 farther back i.e. from the limit at which deposition com- 

 menced. This neutral ground, whereat the depth is just suffi- 

 cient to allow the ice to rest lightly on its own deposit and 

 slide over it without either sweeping it forward or depositing 

 any more upon it, becomes an interesting critical region, sub- 

 ject to continuous forward extension during the lifetime of the 

 glacier, as the deposition beyond it must continually raise the 

 sea-bottom until it reaches the critical depth at which the de- 

 position must cease. This would constitute what I may desig- 

 nate the normal depth of the glaciated sea, or the depth toward 

 which it would be continually tending, during a great glacial 

 epoch, by the formation of a submarine bank or plain of 

 glacial deposit, 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 evident 

 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. Neither can it 

 correspond to the glacial silt which is washed away and sepa- 

 rated from these larger fragments by glacial streams, and de- 

 posited at the outspreadings of glacial torrents 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 glacier 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 composi- 

 tion 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 obtains in modern Alpine 



