52 



University of California Publications. 



[Geology 



of the Virgin Valley Beds is in general that of a fairly open 

 country. 



In the Thousand Creek Beds, representing a later period 

 than the lower portion of the Virgin Valley section, no evidence 

 of lignitic deposits like those of Virgin Valley has been noted. 

 During this epoch the mammalian fauna included many forms 

 which are commonly found on open plains, as antelopes, 

 camels, horses, and rhinoceroses. It is probable that at this 

 time the conditions here were not greatly different from those 

 obtaining in the less arid areas of the northern part of the 

 Great Basin at the present day. 



SUMMAEY OF PRINCIPAL EVENTS IN THE GEOLOGIC HISTORY 

 OP THE VIRGIN VALLEY REGION. 



Following is the succession of principal events in the deposi- 

 tional and erosional history of the Virgin Valley region. The 

 Thousand Creek Beds have been omitted from this scheme, as 

 their position in the column is largely determined by palaeon- 

 tologic data which are naturally presented in the second part 

 of this paper. As the geologic studies presented here have been 

 undertaken for the purpose of assisting in an understanding of 

 the history of mammalian life, in the following table the evidence 

 used is that obtained without the aid of palaeontology. In the 

 general discussion of age and relationships of the faunas, which 

 will appear in Part II of this paper, the evidence of age has 

 been assembled from all sources. 



Terrace formations. Terracing, and accumulation of gravels in later 



history of Virgin Valley. 



Canon cutting Cutting of Virgin Valley to depth of nearly 1500 



ft. 



Mesa Basalt Outpouring of wide-spread but thin sheet of basalt 



Virgin Valley Beds Accumulation of at least 1500 ft. of ash, tuff, and 

 carbonaceous shale. 



Epoch of erosion Erosion probably accompanied by faulting 



Pueblo Range Series Accumulation of much more than 1000 ft. of 

 basalt, tuff, and rhyolite. Rhyolite following 

 basalt. 



