FINAL EESULTS OP EROSION AND UPLIFTING. 23 



fauna. This formation once extended over the Grand Cafion area prob- 

 ably as far south as the river itself, and possibly farther, but has all been 

 swept away as far north as the southern end of the district of High 

 Plateaus. From the region east of the High Plateaus also very large areas 

 of it have been removed. The Upper Trias has also been greatly denuded, 

 and the Lower Trias nearly as much so. The erosion of the Carboniferous 

 has been small, being confined chiefly to the cutting of canons— most 

 notably the Grand and Marble Canons, which are sunk wholly in that 

 series, and in several places have been cut through the entire Palaeozoic 

 series system. 



The average denudation of the Plateau Province since the closing 

 periods of the local Eocene can be approximately estimated, and cannot fall 

 much below 6,000 feet,* and may, nay, probably does, slightly exceed that 

 amount. Of course this amount varies enormously, being in some locali- 

 ties practically nothing and in others nearly or quite 12, COO feet. It is a 

 minimum in the High Plateaus Within that district the average denuda- 

 tion will fall much below 1,000 feet in the sedimentary beds. The enor- 

 mous floods of volcanic emanations have protected them, and these have 

 borne the brunt of erosion, and their degradation has given rise to local 

 accumulations of sub-aerial conglomerates in all the valleys and plains sur- 

 roimding the volcanic areas, thus increasing the protection. 



The general cause which has enabled these strata to survive within the 

 limits of the High Plateaus while they have been so terribly wasted else- 

 whei'e may be stated briefly. Until near the close of the Pliocene the High 

 Plateaus were not only the theatre of an extended vulcanism, but those 

 portions which never were sheeted over by lavas were low-lying areas, 

 where alluvial strata tended to accumulate. They remained, in fact, base 

 levels of erosion during the greater part of Tertiary time. 



Turning now to the Great Basin, which lies even lower than the mean 

 level of the Plateau Country, we find that the pre-eminence of the High 

 Plateaus is due to a totally different cause. Here the difference of altitude 

 is due altogether to difierences in the amounts of uplifting. Since the 



*My owTi estiiiiato exceeds by a few Inindrcil feet that of Professor Powell and also that of Mr. 

 Gilbert. The latter places it at about 5,500 feet. 



