502 JOHN LYON RICH 



line between the gravels and the lowland. At Silver City a tongue 

 of the gravel plateau extends with accordant grade directly across 

 the line of any fault which might have uplifted the gravels farther 

 east. Further conclusive evidence of the lack of fault relation- 

 ships of the lowland is furnished by the fact that the contact 

 between the gravels and the underlying rock runs down the valleys 

 and up across the interstream ridges in a perfectly normal manner, 

 and with such wide divergence from a straight line as to preclude 

 the possibility of an explanation by faulting. 



The failure of the first two hypotheses to account for the inner 

 lowland leaves only the third, that of differential erosion. This 

 calls for, first, the formation of the plateau as a Piedmont plain of 

 accumulation at the base of the mountains; later, the cessation of 

 active aggradation, possibly because of a lowering of the moun- 

 tains through erosion; the initiation of a degradational phase of 

 activity; and finally, the gradual erosional retreat of the mountain 

 front and the reduction of the intermediate land at a rate faster 

 than that of the gravels, leaving them standing in their present 

 relations. 



Important in this connection is the nature of the rock composing 

 the lowland. It is, in the main, a series of soft Cretaceous shales 

 cut by dikes of a moderately resistant igneous rock. In parts of 

 the lowland the shales are absent and the bed-rock is igneous. 

 This, however, makes little difference in the nature of the resulting 

 topography. Everything is worn down to a nearly uniform level 

 lower than that of the gravels. 



A discussion of all the possible causes of the change in the phase 

 of activity from one of aggradation to one of degradation would 

 be out of place here. Two such may, however, be mentioned. The 

 first is change in climate, the second, a lowering of the mountains 

 by erosion with consequent relative increase in the factor of decom- 

 position, over that of disintegration and transportation, brought 

 about by the lessened slope. 



If the same process of differential erosion continues, the moun- 

 tains will eventually become much reduced in height while the 

 gravels, suffering less by erosion, will stand relatively higher and 

 may finally come to dominate the topography of the surrounding 



