'228 



University of California Publications. 



[Geology 



contact below the rhyolitie gravels there does not appear to be a 

 noticeable difference in position between the upper and lower 

 sedimentary beds below the Mesa Basalt; at any rate no such 

 difference appears as would be produced if any considerable 

 •change in the topography had developed through faulting or 

 other movements. 



On any hypothesis excepting that the Thousand Creek Beds 

 were formed by accumulation late in the history of the cutting of 

 Virgin Valley, it would be impossible to avoid the conclusion that 

 important faulting movements have occurred in this region in 

 post-Thousand-Creek time. 



Relation of Thousand Creek Beds to other Formations of the 

 Pacific Coast and Basin Regions. — The unique character of- the 

 mammalian fauna found in the beds at Thousand Creek, and 

 the imperfectly understood stratigraphic relations of the forma- 

 tion in which this fauna occurs make it difficult to estimate the 

 position of the Thousand Creek Beds in the scheme of Pacific 

 Coast formations. A possible relationship to the Rattlesnake 

 Beds of the John Day Region in Oregon is the correlation which 

 naturally suggests itself before any other. Correlation with 

 other formations is also suggested, but the basis for comparison 

 is very slight. 



The type exposure of the Rattlesnake fortunately occurs in 

 the same region with the typical section of the Mascall Miocene, 

 and with well-marked Pleistocene deposits, so that the earlier 

 and later limits of age of the Rattlesnake are quite clearly 

 defined. 



The typical Rattlesnake Beds rest in marked unconformity 

 upon the Mascall along the border of the Blue Mountains in 

 the vicinity of Dayville on the John Day River. The Mascall 

 here occupies a trough formed on the north side by the Colum- 

 bia Lava dipping to the south, and on the southern side by the 

 mass of the Blue Mountains, the Columbia Lava being faulted 

 or sharply folded against the northern side of this ridge of 

 the mountains. The Mascall Beds agree in dip and strike, so 

 far as observed, with the underlying Columbia Lava, and were 

 deposited previous to the movement expressed in the sharp 

 deformation of the lava. As the exposures of the Mascall are 



