TERTIARY FORMATIONS. 



205 



or three species of fresli-water mollusca seem to sustain his view, but the 

 fresh-water forms of the Plateau Province so frequently have a very great 

 vertical range, that they are apt to mislead in just such cases, and require 

 collateral evidence to justify such a conclusion. On the other hand, there 

 is no indication in the appearance of the rocks of such a break of the con- 

 tinuity, and the whole of the Tertiary here exposed seems to belong to one 

 series without unconformity and without any break in the conditions nec- 

 essary to continuous deposition. It has, therefore, seemed to me unadvis- 

 able to intercalate a vacant horizon in a series which to all appearances is 

 continuous. 



The white limestone at the summit of the formation is a very con- 

 spicuous member and forms the surface of the plateau for a considerable 

 distance south of Panquitch Lake, where it is laid open by ravines and 

 exposed in buttes capped by basalt. It reaches a thickness of rather more 

 than 300 feet in some places, but is usually much less. It is very impure; 

 sometimes very siliceous, holding agate or chalcedony, and is also some- 

 times marly. The total thickness of the Eocene beds is from 1,100 to 

 1,200 feet. 



The epoch of final emergence from the lacustrine condition seems to 

 have been earlier here in the southwestern part of the Plateau Province 

 than in the middle or northern portions. This is indicated by the earlier 

 age of the most recent lacustrine beds; for as we proceed northward later 

 and later members gradually make their appearance. In the south, not 

 more than the lower third of the Eocene is present; in the middle district, 

 barely more than one-half; while around the southern slopes of the 

 Uintas nearly or perhaps quite the whole of it is revealed. It may be con- 

 jectured that the Lower Green River beds once existed here and were 

 eroded and wholly removed before the volcanic eruptions began. This 

 cannot be wholly disproven, but the view is extremely improbable; for in 

 the epoch immediately following the final emergence the conditions were 

 not favorable to a rapid erosion; the region was not at that time an elevated 

 one; it could scarcely have exceeded a few hundred feet in altitude above 

 sea level, and there were no important displacements nor dislocations. 

 The Bitter Creek beds cover many hundred square miles of continuous 



