46 



University of California Publications. 



[Geology 



Thousand Creek, the mammalian fauna which is found widely 

 spread in the exposures of this region represents a stage of the 

 late Tertiary, but apparently not the very latest portion of the 

 Tertiary. 



In comparing the fauna of Thousand Creek with that of the 

 Virgin Valley Beds, there are found to be a few species common 

 to the two ; but by far the greater number of the species, and 

 even of the genera, are different. In most respects in which it is 

 possible to make a comparison, the Thousand Creek forms are 

 more advanced or more specialized than those of Virgin Valley. 

 Judging by the fact that a few species are common to the two 

 faunas, there is reason for considering them as not widely 

 separated in time. On the other hand, it is difficult to place 

 them in the same epoch, and it is evident that the Thousand 

 Creek fauna is the later one. 



A summing up of the evidence presented by the Thousand 

 Creek fauna with reference to the age in relation to that of the 

 other formations of this region shows the following points: (1) 

 The Thousand Creek fauna is of late Tertiary age; (2) It is later 

 than the fauna obtained from the lower and middle Virgin 

 Valley Beds ; ( 3 ) It was not widely separated in time from the 

 known Virgin Valley fauna, as there are a few mammalian 

 species common to the two. 



Although resembling the Virgin Valley Beds in a general 

 way, it is evident from their contained fauna that the wide 

 extent of exposures in which collections of fossil mammals have 

 been made in the Thousand Creek region cannot represent the 

 lower or middle portion of the section in Virgin Valley, in which 

 the typical Virgin Valley fauna has been found. Unfortunately 

 almost nothing is known of the fauna from the uppermost por- 

 tion of the Virgin Valley section, possibly because the steep 

 exposures immediately below the basaltic capping present a col- 

 lecting area relatively much smaller than that representing the 

 lower horizons. It is therefore necessary to reckon with the 

 possibility that the exposures at Thousand Creek represent the 

 upper portion of the Virgin Valley section. 



If the Thousand Creek exposures be held to represent the 

 uppermost portion of the Virgin Valley section, the present 



