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University of California Publications. 



| Geology 



Determined according to the range of mammalian genera in 

 North America, the Virgin Valley fauna must be considered as. 

 Middle Miocene. The generic types represented are those com- 

 monly found in the middle and upper divisions of the Miocene. 

 The Lower Miocene is excluded by the advanced stage of develop- 

 ment of the horses represented in Hypohippus and Merychippus, 

 the extreme rarity of oreodonts, and the presence of advanced 

 deer-like forms such as Dromomeryx and Merycodus. The pres- 

 ence of Moropus with a Thinohyus-like form and the absence of* 

 Pliohippus indicate a stage earlier than Upper Miocene. About 

 equal numbers of upper and lower Miocene genera are present, 

 but the Middle Miocene character of the fauna is indicated by 

 the abundance of Hypohippus and Merychippus with the entire 

 exclusion of any forms of the Pliohippus or Protohippus type. 



Relation of Virgin Valley Fauna to its Environment. — The 

 greater part of the mammalian material from Virgin Valley was 

 obtained in the zone which includes the carbonaceous shales and 

 lignite deposits. In this zone diatomaceous deposits are well 

 developed, and fragmentary fish remains are occasionally found. 

 Fossil wood of large conifer-like trees is abundant at many 

 localities in this portion of the section. The nature of the deposit 

 in general indicates that moist ground, swamps, and possibly 

 even considerable bodies of water existed in this region during 

 the period in which the typical Virgin Valley fauna flourished. 

 Concerning the nature of the vegetation we know as yet com- 

 paratively little, as most of the remains obtained were imper- 

 fectly preserved! The plant specimens from the carbonaceous 

 shales include rushes, willows, and a number of other forms not 

 determined. 



The nature of the deposits and of the contained remains in 

 the principal mammal-bearing zone of the Virgin Valley Beds 

 does not necessarily indicate that conditions were then entirely 

 different from those obtaining in this region at the present time. 

 Large marshy areas and lakes of considerable size existing today 

 in the northern Basin region are the habitat of abundant plant 

 and animal life, while the most arid conditions may obtain only 

 a few hundred yards from the water. Considering, however, the 



