182 STUCTURAL GEOLOGY. 



in the background as it would appear had there been no degradation, but 

 uplift only. It is believed that a careful study of the illustration will be 

 amply repaid. 



RATE OF DEGRADATION. 



It would be interesting to know at what rate this degradation has pro- 

 gressed. We know of no means by which an absolute rate can be deter- 

 mined, but there are certain facts which lead to the conclusion that a 

 maximum rate for this region was never established. These facts are 

 found in the character of the topographic features produced by degradation, 

 but in order that we may more fully understand them it will be well to 

 .briefly examine the agencies, methods, and conditions of degradation. 



Degradation consists of disintegration and transportation, and they are 

 mutually dependent parts of the general process. If disintegration is re- 

 tarded, transportation is retarded, for the materials must be furnished ready 

 for transportation. Again, if transportation is retarded, disintegration is re- 

 tarded, as the beds to be disintegrated are to some extent protected by the 

 accumulated products of the process ; and if either is accelerated, the other 

 must be accelerated. 



DISINTEGRATION. 



The rock masses which are brought above the level of the sea by up- 

 heaval are always found to be more or less coherent. Although these rocks 

 are chiefly of sedimentary origin, the exceptions being extravasated masses, 

 still they are usually found to have assumed a more or less crystalline struc- 

 ture, due either to the manner of their deposition or subsequent metamor- 

 phism, so that the minute parts, molecular or mechanical, of which these rocks 

 were originally aggregated, cohere in great masses. There are many degrees 

 of this coherence from bad-land sandstones or shales of extreme friability to 

 schists and granites of extreme coherence; but whatever may be their degree 

 of coherence, they must be disintegrated prior to their transportation to 



the sea. 



PETROLOGY AS RELATED TO DISINTEGRATION. 



The endurance of rocks as determined by their coherence depends on 

 1. Geologic Structure. The rocks may be irregularly massed or strati- 



