506 JOHN LYON RICH 



mountain area. If the streams crossing the Piedmont gravel fans 

 sink deeply enough they may finally cut entirely through the gravel 

 into the underlying rock. In that case we will have a plateau 

 between the streams, capped by gravels of a composition corre- 

 sponding to that of the rocks of the lowland which occupies the site 

 of the original mountains, but lying at a level higher than the 

 summits of these mountains as they now exist. 



Various combinations of factors will modify in different ways 

 the course of development as sketched above, but the general 

 principle involved should hold true, and the results should be in 

 harmony with this principle as modified by the particular factors 

 dominating in any one case. 



EXAMPLES ILLUSTRATING THE TYPE OF DEVELOPMENT ABOVE 



OUTLINED 



As examples of the influence of the slower differential erosion 

 of gravel deposits the following may be mentioned: The region 

 east of Silver City; the Uinta Mountains and the associated Bishop 

 conglomerate, both described in the preceding pages. The Cats- 

 kill Mountains of New York, in their relation to the old lowland to 

 the east, are a possible illustration of the principle. 



