14 INTRODUCTORY. 



But if we are to admit that the strata sank as rapidly as they accu- 

 mulated we cannot shake off some ulterior questions. By virtue of what 

 condition of the underlying magmas was such a subsidence possible? If 

 they sank, they must have displaced matter beneath them, and what became 

 of the displaced matter? If we look around the borders of the area and 

 partially within it, we shall find a problem of an inverse order. The Uintas, 

 the Wasatch, the Great Basin have suffered an amount of degradation by 

 erosion, which is perhaps one of the most impressive facts which the physi- 

 cal geologist has yet been brought to contemplate. From the Uintas more 

 than 30,000 feet of strata have been removed since their emergence. From 

 the Wasatch the removal has been much more; from the Great Basin the 

 degradation has been many, we know not how many, thousand feet. We 

 are not prepared to believe that the Uintas ever stood 8 miles high, nor 

 the Wasatch 12 miles high, but we know that their altitudes are merely 

 the difference between elevation and erosion. It was from these ranges 

 that the heaviest masses of the Cretaceous-Eocene sediments were derived. 

 As fast as, or even faster than, the mountains were devastated to supply 

 mass for the new strata, the}" continued to rise. But if they rose, fresh matter 

 must have been thrust under their foundations, replacing the rising strata. 

 Whence came the replacing matter ? It may be premature as yet to say 

 that the elevation of the mountains and subsidence of the strata are cor- 

 related in the way which these inquiries suggest, but the juxtaposition of 

 the facts must be regarded as significant. 



POST-EOCENE HISTORY EROSION. 



With the desiccation of the Eocene lake began a new order of events 

 in the history of the Plateau Country; in truth, its most instructive and 

 impressive chapter. The lessons which may be learned from this region 

 are many, but the grandest lesson which it teaches is Erosion. It is one 

 which is taught, indeed, by every land on earth, but nowhere so clearly as 

 here. If we could but find the evidence, we might be able in other regions 

 to point to erosions of much greater amount. We may suspect that in the 

 Appalachians a denudation has occurred compared with which the denuda- 

 tion of the Plateaus is small ; and such an inference has no intrinsic 



