56 0. P. Hay — Deposit of the Glacial Drift. 



in the Drift. In some cases such deposits may have been pro- 

 tected from glacial action by thick accumulations of compacted 

 snow and ice which heralded the approach of the glacier. 



While we may justly, I think, consider the eroding action of 

 a glacier, like that of a running stream, to be some function of 

 its velocit}'', the same cannot be said of the transporting and 

 depositing powers of the glacier. When the velocity of the 

 stream is reduced even a little, a portion of the materials it 

 bears along may be deposited ; but stones, large and small, 

 once received%into the bosom of the glacier are borne along, 

 whatever may be the velocity. Yet we have evidences which 

 show that in some cases bowlders 'have sunken in the glacial 

 ice, so that they have, as Prof. J. D. Dana relates, got into a 

 lower current running in a different direction. This is no 

 more than we might expect of heavy rocks that are supported 

 in a mass of ice which is constantly undergoing incipient lique- 

 faction and regelation ; and we may suppose that this tendency 

 of rocks to gravitate downward would, in a slowly moving 

 glacier, prevail over the tendency of the intestinal motion to 

 throw them upward. However, it is not probable that these 

 movements have had much to do with the making of the Drift 

 deposits, to account for the enormous mass of which we must 

 have recourse to other facts and principles. 



What is known as the law of differential motion prevails in 

 all glaciers. This law is a statement of the fact that different 

 portions of the glacial stream move with very different veloci- 

 ties, the upper portions faster than the lower, the middle of the 

 stream faster than the edges. The velocity of flow of the 

 lower parts must be affected greatly by the character of the 

 bed, and by the relative quantities of foreign materials enclosed. 



There is another fact which must be considered at this point, 

 namely, that there is in all glaciers more or less melting going 

 on at the bottom. The sun's rays may cause the surface of the 

 ice to thaw and waste away, but the heat that escapes from the 

 earth must expend its energy in melting the bottom of the 

 glacier. This action of the terrestrial heat is recognized by 

 glacialists, and produces what is known as subsidence of the 

 glacier. Accordingly we find streams of water escaping from 

 beneath even the glaciers of the polar regions. The inevitable 

 result of this melting of the lower layers of a slowly moving 

 ice-stream must be to cause the rocks, sand and clay to accum- 

 ulate in greater proportions in its lower parts. If we now con- 

 nect this conclusion with the law of differential motion, we 

 shall, I think, be able to account for the deposit known as the 

 bowlder clay. While the great glacial ice-sheet of North 

 America was descending from the Laurentian Highlands, where 

 it took its origin, its weight was so enormous and its velocity 



