DENUDATION, OR GENERAL EROSION. 253 



geological time ; then of the rate of rain and river erosion ; 

 and then a rough estimate of the time necessary to do the 

 work. 



Modes of determining Amount of Denudation. 



There are many ways in which geologists determine the 

 amount of denudation. In case of faults, as in Fig. 158,, 



FIG. 158. 



in which the strong line, a a, represents actual surface, 

 there must have been great erosion to obliterate all sur- 

 face indications of the slip. Now, there are cases of slips 

 20,000 feet vertical, as in Pennsylvania and on the north 

 side of Uintah, in which surface indications are entirely 

 removed by erosion. Again, in case of isolated erosion- 

 peaks, like Fig. 159, it is evident that the whole interven- 



SOOOJt 



FIG. 159. Denudation of red sandstone, northwest coast of Ross-shire, Scotland. 



ing country has been carried away. Now, such peaks are 

 often 2,000 to 3,000 feet high. But the most universal 

 means of estimating the amount of erosion is by resto- 

 ration of folded strata. This is shown in Fig. 160, and in 

 iiKiuy of the preceding figures on mountains. 



By all these methods it has been estimated by British 



