Studies m the Sierra 



were not thus derived from 

 the summit peaks overtopping 

 the glaciers, but from the 

 rocks past which and over 

 which they flowed. I have seen 

 the north Mount Ritter Gla- 

 cier and many of the glaciers 

 of Alaska in the act of grind- 

 ing the side of their channels, 

 and breaking off fragments 

 and rounding their angles by 

 crushing and rolling them be- 

 tween the wall and ice. In all 

 the pathways of the ancient 

 glaciers, also, there remain no- 

 ble illustrations of the power 

 of ice, not only in wearing 

 away the sides of their chan- 

 nels in the form of mud, but 

 in breaking them up into huge ? 

 blocks. Explorers into the up- " 

 per portion of the middle 

 granite region will frequently 

 come upon blocks of great 

 size and regularity of form, 

 possessing some character of 

 color or composition which 

 enables them to follow back 

 on their trail and discover the 

 rock or mountain - side from 

 which they were torn. The 

 size of the blocks, their abun- 

 dance along the line of dis- 

 persal, and the probable rate 

 of motion of the glacier which 

 quarried and transported them, 

 form data by which some ap- 

 proximation to the rate of this 

 sort of denudation may be 



311 



