30 



University of California Publications. 



[Geology 



of round erinoid stems. Although the crinoid fragments are 

 insufficient evidence upon which to base a definite age determi- 

 nation, the presence of stems of this character probably indicates 

 that the beds are older than the Triassic, as the most abundant 

 Triassic crinoids of this general region are not of the round- 

 stemmed type. The presumption is that the limestones are 

 Carboniferous, though there is no definite evidence that they 

 are not older. 



The sides of the Pine Forest Range are flanked by series 

 of igneous rocks that apparently correspond in a large part to 

 the Pueblo Range Series represented by such extensive exposures 

 on the western side of the Pueblo Range immediately to the 

 north. 



VIEGIN VALLEY SECTION. 



The geologic section exposed in Virgin Valley is the most 

 important examined, as the larger part of the known Tertiary 

 history of this region is illustrated here in excellent exposures. 

 At least three formations are represented, of which the middle 

 one is the mammal-bearing series known as the Virgin Valley 

 Beds. The Canon Rhyolite immediately underlying the mammal 

 beds is presumably of the same age as the upper or rhyolitic 

 portions of the Pueblo Range Series. The Mesa Basalt, which 

 overlies the Virgin Valley Beds, is widely spread over the sur- 

 rounding region. (See fig. 2). 



MESA 

 BASSIST 



A 



SECTION A-B 



Fig. 2. — Section along A-B line on plate 2, showing section across 

 Virgin Valley and Thousand Creek Ridge. 



The structure of the beds in Virgin Valley is in general 

 synclinal. The basal formation, the Canon Rhyolite, dips south- 

 west and beneath the valley from Thousand Creek Ridge to the 

 east; and reappears, dipping northeast, in Hard Rock Ridge to 

 the southwest. The Virgin Valley Beds form a shallow trough, 



