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University of California Publications. 



[Geology 



(3) Post-Mesa-Basalt; formed from older wash of Virgin 

 Valley, faulted down. 



(4) Post-Mesa-Basalt; formed, from younger wash of Virgin 

 Valley, not moved far by faulting. 



(5) Composite, partly Virgin Valley and partly Pleistocene. 

 When cheeked by what we know of the faunal relationships 



of the Virgin Valley and Thousand Creek Beds, it is noted that 

 the interval between the Virgin Valley and Thousand Creek 

 faunas seems to amount to a period at least as long as the Upper 

 Miocene, and that the Thousand Creek Beds cannot be later than 

 early Pliocene. This would make it improbable that the Thous- 

 and Creek Beds belong to the same period of deposition as the 

 Virgin Valley. On the other hand it seems improbable that the 

 cutting of the present canon of Thousand Creek was well under 

 way before the beginning of Pliocene time. These suggestions 

 seem to narrow the problem down to the following possibilities : 



(1) That the Mesa Basalt is Miocene in age and that the 

 present canons began to cut in early Pliocene time, the Thousand 

 Creek Beds being formed by the accumulations of early wash 

 from this erosion. 



(2) That the Thousand Creek Beds represent a p re-Mesa 

 Basalt formation of considerably later age than the principal 

 mammal zone of the Virgin Valley Beds. 



According to the first view the Thousand Creek Beds are 

 post-Mesa-Basalt in age, and were accumulated during the cut- 

 ting of Virgin Valley or other valleys of approximately the same 

 age. There are several arguments which may be put forward 

 in support of this view, but it seems especially desirable to have 

 more evidence regarding the relation of the northern and west- 

 ern extensions of the Thousand Creek Beds to the Mesa Basalt 

 before it can be seriously considered. 



The second view postulates the pre-Mesa-Basalt age of the 

 Thousand Creek Beds and makes them either the equivalent of 

 the uppermost portion of the Virgin Valley section or a pre- 

 Mesa-Basalt accumulation formed from the erosion of the Virgin 

 Valley and not represented in the portion of the Virgin Valley 

 section examined. The fact that the upper portion of the Virgin 



