142 R. Pumpelly—Secular Rock-disintegration. 
on the rocks of the peaks and precipices that rise above the ice. 
But a continental glacier, such as covered Northeastern Amer- 
ica, had no such source of supply; for all the mountains were 
covered with ice and were thus protected from the operation of 
this process. 
The great ice-sheet removed the residuary accumulation of 
loose material down to the solid rock; in doing this the loose 
ial was ground together, producing much clay and sand; 
and besides this, the glacier grinding with this material under 
the immense weight of the ice on the solid rock, exerted an 
s 
all the bowlders of the ground moraine have been abraded 
in nearly the ratios of their respective surface areas ' 
Not the least important suggestion is the influence of the 
causes we have considered on the suceession of marine strata. 
Great areas like Southern China and the southeastern Unt- 
ted States have remained as dry land from the Triassic peri 
till now. Throughout this long lapse of time, disintegration 
and decomposition have been at work, and one has only to loo 
at the excavations in Philadelphia and on the railroads running 
south from there to get some idea of what these agencies can 
accomplish. But the abundant vegetation of a moist, periphe- 
region has so thoroughly protected the residuary mass from 
erosion that the streams run clear, carrying to the ocean only 
