158 GEOLOGY OF THE HIGH PLATEAUS. 



of land from the water, or whether it merely indicates a local exposure to 

 denudation, it is not possible at present to say. 



TERTIARY LACUSTRINE FORMATIONS 



The history of the Plateau Country which is at present best known is 

 the history of its Tertiary formations. This remains to be written; but 

 materials for it have been widely collated, and are in the possession of Pro- 

 fessor Powell, who will, it is believed, discuss the subject at an early day. 

 A more promising and instructive one probably is not to be found in the 

 entire range of North American geology. Nothing- more is needed here 

 than a mere summary, which may serve as a guide and index to the mean- 

 ing of the terms employed in this monograph. 



The Tertiary system of the Plateau Country is lacustrine throughout, 

 with the exception of a few layers near the base of the series, which have 

 yielded estuarine fossils. The widely varying strata were accumulated 

 upon the bottom of a lake of vast dimensions, and were derived from the 

 waste of mainlands and mountain platforms, some of which are still dis- 

 cernible. The region of maximum deposit was in the vicinity of the 

 Wasatch and Uintas, where in the course of Eocene time more than 8,000 

 feet of beds were laid down. As we proceed southward, these heavy de- 

 posits attenuate, parti 3^ hy a diminution in the thickness of the individual 

 members and partly because the period of deposition ceased earlier the 

 farther southward we go, until in the southern part of the province only 

 the lower Eocene is found, or, indeed, was ever deposited. The High 

 Plateaus occupy the belt through which this diminishing bulk and successive 

 elimination of upper members is well seen. In the Wasatch Plateau, at the 

 extreme northern part of the district, we find the two lower divisions of the 

 Eocene present in great volume; and in the valley of the Sevier and San 

 Pete we find what is undoubtedly a still higher division. At the southern 

 portion of the district only the lower division can be clearly made out, 

 though some of the upper beds may prove to belong to a later period. The 

 present weight of evidence, however, seems to me to place them in one divis- 

 ion, the "Bitter Creek" of Powell. 



In the southern plateaus, the Markagunt and Paunsagunt, we find 



