Vol. 6] Merriam: Virgin Valley and Thousand Creek. 209 



general persistence of moist conditions, and the nature of the 

 vegetation indicated in this section, the weight of evidence indi- 

 cates that the climate was more humid and somewhat warmer 

 than at present. The vegetation suggests also that the altitude 

 was probably less than 5000 feet, which is now approximately the 

 level of the mammal zone in Virgin Valley. 



As nearly as can be judged from our present knowledge, the 

 Virgin Valley Beds were laid down over a region in which fault- 

 ing had already produced a certain degree of relief. As the 

 deposition progressed, the irregularities of topography were 

 gradually smoothed over by filling in of the depressions, until 

 plains or shallow lakes of wide extent had been developed. 



The nature of the mammalian fauna occupying the Virgin 

 Valley region in Middle Miocene time accords well with what one 

 might expect in such an environment. Probassariscus among the 

 carnivores, Aplodontia of the rodents, and the brachyodont 

 Hypohippus of the ungulates suggest a region containing wooded 

 areas. Merychippus and Merycodus, with long-crowned grazing 

 teeth, suggest the open plains. As nearly as we can determine, 

 both kinds of environment were available, and in localities so 

 near together that remains representing the two types of faunas 

 might readily be mingled in accumulations forming over the 

 lowest areas of the region. 



FAUNA OF HIGH ROCK CANON AND LITTLE HIGH 

 ROCK CANON 

 The exposures at High Rock Canon and Little High Rock 

 Canon resemble those of Virgin Valley in many respects. The 

 collections made at these localities comprise the following forms : 

 High Bock Canon Little High Rock Canon 



Tephrocyon ( 1), compare mrestris (Condon). Moropus ( ?) , sp. 



Tephrocyon ( ?) , sp. o. 



Aelurodon ( '?) , sj>. 



Merychippus isonesus (Cope). 



Merychippus, near seversus (Cope). 



Aphelops{ ?) , sp. 



Moropus { 't) , sp. 



Mastodon (Tetrabelodon "?, sp.). 



Blastomeryx mollis, n. sp. 



Merycodus nevadensis, n. sp. 



