36 



University of California Publications. 



[Geology 



are in this portion of the section a number of beds of gravel 

 and boulders which are evidently of fluviatile origin, but the 

 impression given by this section as a whole is that it is largely 

 of aeolian origin. This suggestion is also supported by the 

 nature of the fossil remains in these beds, which are those of 

 land forms. This does not preclude the possibility that some 

 of the strata accumulated in temporary lakes. 



During the deposition of the Virgin Valley Beds the con- 

 ditions of accumulation were apparently much as those at the 

 present time in most of the large valleys of the Nevada or eastern 

 Oregon region. The Canon Rhyolite evidently formed the rim 

 of the valley, while the sediments laid down in it were spread 

 out to form a broad and nearly level floor. Whether the 

 conditions were such as to cause sedimentation in water or in 

 air, the evenness of the stratification remained much the same. 



As suggested under the discussion of the later history of 

 this region (see p. 43) there is reason for believing that a 

 considerable thickness of rhyolitic gravels resting unconformably 

 upon the middle beds of the Virgin Valley section in the angle 

 between the valleys of Virgin Creek and Beet Creek (pi. 8), 

 represents deposition within the Virgin Valley epoch. "Whether 

 this unconformity exhibited here is general throughout the 

 Virgin Valley section, or whether it is a purely local feature is 

 not known. If it should be found to represent a widespread 

 condition of erosion, it would be necessary to divide the Virgin 

 Valley section into an upper and a lower division. In this case, 

 the name Virgin Valley Series may be applied to the whole 

 group of beds between the Canon Rhyolite and the Mesa Basalt. 

 The beds below the unconformity would then be known as the 

 Lower Virgin Valley Beds, those above the unconformity the 

 Upper Virgin Valley Beds. 



Mesa Basalt. — AVhere they are not uncovered by erosion, the 

 Virgin Valley Beds are capped by an extensive sheet of olivine 

 basalt of a doleritic facies. This capping forms the "rim rock" 

 of the great mesas on both sides of the valley of Virgin Creek 

 and may be known as the Mesa Basalt (see pis. 9 and 10, 

 text fig. 2, p. 30, and text fig. 3). So far as observed, the 

 lava sheet is not distinctly unconformable upon the underlying 



