860 GEOLOGY 



were capable of planing and beveling and striatin^ many stones, 

 especially the softer ones of the unst ratified drift, while rounding 

 and leaving unstriated most of those of the stratified; but that the 

 agency or agencies concerned must have been such that under 

 certain circumstances their activities failed, on the one hand, to 

 leave more than a very small percentage of the stones of the un- 

 stratified drift beveled and striated, while, on the other hand, they 

 sometimes permitted the stratification of gravels containing many 

 sub-angular, plane-faced, and striated stones, varying in size from 

 pebbles to bowlders. From the strise on the bed-rock beneath the 

 drift and the unweathered character of the surface of the rock, it is 

 clear that severe wear was inflicted on the surfaces over which the 

 drift was spread, while the positions in which the stria? were devel- 



Fig. 573. Diagram to show the effect of ice wear on slight depressions in the 



surface of rock. 



oped show that the agency which inflicted the wear was able to 

 adapt itself to all sorts of surfaces. The general parallelism of strise 

 in a limited area, and the systematic departure from parallelism 

 over great areas, are also significant of the manner in which they 

 were produced. From the topography of the drift it is known that 

 the forces which produced it must have been such as were able to 

 develop plane surfaces at some points, surfaces marked by more or 

 less symmetrical drift-hills, which are measurably independent of 

 rock-topography at others, and short, choppy hills, associated with 

 undrained depressions, in still others. 



The true theory of the drift must explain all these facts and 

 relations. Any hypothesis which fails to explain them all must be 

 incomplete at the least, and any hypothesis with which these facts 

 and relations are inconsistent, must be false. 



Geologists are now very generally agreed that glacier ice, sup- 

 plemented by those other agencies which glacier ice calls into IKMH^, 

 is the only agent which could have produced the drift. But it is 





