GLACIAL EROSION AND TRANSPORTATION. 261 



small it was. But Professor Newberry well replied that, if 

 there was that amount of erosion so near the margin, what 

 must it not have been farther back, where the stream of ice 

 had acted foi- an indefinitely longer time. Probably, how- 

 ever, Newberry is extravagant when he estimates that farther 

 north the ice w T as ten times as thick, and continued to act 

 ten times as long, making its erosive power one hundred 

 times as great as that near the Water-Gap.* 



The foregoing evidence of glacial erosion drawn from 

 the extent of marginal glacial deposits is complicated by our 

 ignorance of the extent to which disintegration of the rocks 

 had proceeded before the Glacial period. Professor Whit- 

 ney f and Mr. Pumpelly \ have specially pressed this point, 

 as have Professor S terry Hunt and the late Mr. L. S. Bur- 

 bank,* to whom more credit is due than he has generally 

 received for his early and sagacious suggestions upon the 

 subject. The contrast between the glaciated and the un- 



Fig. 77.— Ideal section showing result of disintegration in an unslaciated region. 

 (Chamberlin.) 



glaciated region, in the extent to which the surface rocks are 

 disintegrated by subaerial agencies, is very striking. South 

 of the glaciated region granitic masses and strata of gneiss 



* " School of Mines Quarterly," p. 12 

 f " Climatic Changes," p. 7 et aeq. 



% " The Relation of Secular Rock-Disintegration to Loess, Glacial Drift, and 

 Rock Basins," in the "American Journal of Science," vol. cxvii, 1879, p. 133 

 et neq. 



* " On the Formation of Bowlders and the Origin of Drift Material," in 

 the " Proceedings of the Boston Society of Natural History," vol. xvi, Novem- 

 ber 19, 1873. 



