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



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sions, due to crustal movements of less magnitude than those 

 which produced the deep, broad valleys to the east ; or they are 

 comparatively narrow canons due to erosion. The general level 

 of this region is much higher than that of the broad valleys to 

 the east. 



The region under consideration may be classified, as a whole, 

 as semi-arid, excepting some of the highest zones of the larger 

 mountain masses. Over the greater part of the area sage-brush 

 is the dominant type of vegetation. Trees are almost entirely 

 absent, excepting scattered junipers on the hills, a few alders 

 and willows along the streams, and a few pines in the highest zone. 



Notwithstanding the generally arid nature of the country, 

 there is sufficient grass, and other vegetation which may serve as 

 food for herbivorous animals, to support a large mammalian 

 population. In comparatively recent time the ungulates have 

 been quite abundantly represented by prong-horn antelope, deer, 

 and mountain sheep. At the present day the carnivores are num- 

 erously represented by coyotes, wild-eats, and badgers, and an 

 abundant rodent population includes many genera and species. 

 The fauna as a whole is surprisingly rich in variety of forms and 

 number of individuals. 



GEOLOGIC FEATURES OP THE REGION. 



The geologic features of the region visited by the expedition 

 present a most attractive study. A considerable number of 

 well-defined formations are represented and numerous in- 

 structive sections are exposed by extensive fault-scarps and 

 deeply eroded canons. The district immediately surrounding 

 the field examined apparently shows a range of geological systems 

 extending from Palaeozoic to Quaternary. 



As the primary object of the expedition was to obtain a 

 representation of the mammalian fauna of the sedimentary 

 formations in the Virgin Valley region, the acquisition of 

 palaeontological material was the occupation of first importance. 

 Investigation of the formations represented has therefore of 

 necessity been confined almost exclusively to the fossil-bearing 

 beds with those adjacent to them, and may not be considered as 

 more than a reconnaissance. 



