254 THE ICE AGE IN NORTH AMERICA. 



free from bowlders, except in the lines of movement extend- 

 ing from the nwnataks. A sufficient reply to this objection 

 is to say that a country so long subject to glacial movements 

 as Greenland has been must already have had the looser frag- 

 ments of rock, which can be incorporated into the ice, gath- 

 ered up and removed to the front, so that now the floor of 

 the whole continent is so smooth and bare of fragments that 

 there is nothing left for the glacier to get hold of. But dur- 

 ing the movement of the glacier over the northern part of 

 the United States, everything favored the mode of trans- 

 portation and elevation of bowlders as described above. 



As already shown,* the transportation of large bowlders, 

 though impressive in view of the enormous masses moved, 

 probably represents but an insignificant part of the work of 

 erosion and transportation which took place during the Gla- 

 cial period. Some would even regard the subglacial streams 

 pouring out in front of every glacier, and surcharged with 

 fine sediment, as the most important instruments of glacial 

 transportation. How much of the glacial grist has been car- 

 ried away by this process it is impossible to determine with 

 accuracy. It will, however, be of some interest to learn the 

 best attainable results. 



In 1864 Dolfus-Ausset measured the fine sediment carried 

 out by the stream from below the Unter-Aar Glacier, finding 

 132 grammes in a cubic metre of water. This corresponds to 

 a yearly rubbing off of about 0*6 millimetre of rock from under 

 all parts of the glacier's basins, or an erosion of one metre in 

 1,666 years — about two and a half times as much as water 

 could do in the same period. It is not determined how much 

 of the sediment came from sand washed under the ice by side 

 streams, but it shows that the glacier does a considerable 

 amount of work (" Materiaux pour l'fitude des Glaciers, " 1864, 

 vol. i, p. 276). f 



* See above, p. 136. 



f Professor W. M. Davis, in " Proceedings of the Boston Society of Natural 

 History," vol. xxii, 1882, p. 26. 



