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



231 



are the, (1) Virgin Valley Beds, corresponding' to the Maseall ; 

 and (2), the great valleys, originating, like the valley of the 

 John Day, through geologically recent erosion. If the relation of 

 the Thousand Creek Beds to both of these factors were clearly 

 shown, important evidence would be available for checking the 

 relative ages of the two formations. 



If the Thousand Creek Beds are pre-Mesa-Basalt, as seems 

 possible, they may correspond closely in age to the Rattlesnake. 

 It is perhaps worth noting that the remarkable extent of the 

 layer of Mesa Basalt in the Virgin Valley region is paralleled, 

 in a manner, by the great extent of the bed of Mesa Rhyolite, 

 forming the mesa capping over a considerable part of the Rat- 

 tlesnake. 



If the Thousand Creek Beds are post-Mesa-Basalt they are 

 either younger than the Rattlesnake or the canon-cutting was 

 initiated at an earlier date than in the region to the north of 

 the Blue Mountains. As nearly as we are able to judge there 

 seems reason to believe that the great canons of the entire region 

 under consideration owe their origin to an uplift of continental 

 character, which occurred near the close of Pliocene time, and 

 unless special conditions have been introduced in one or the 

 other of the regions discussed we are presumably near the truth 

 in considering the canon-cutting as nearly coincident in the 

 two areas. In that case post-Mesa Basalt age of the Thousand 

 Creek Beds would place them at a much later date than the Rat- 

 tlesnake. It would also evidently place them within the limits 

 of Pleistocene time, which is clearly negatived by their fauna. 

 It seems, therefore, that with the evidence at hand there is rea- 

 son for considering the Thousand Creek Beds as pre-Mesa 

 Basalt. 



As will be seen, the lines, however drawn, seem to indicate 

 that the Rattlesnake and Thousand Creek epochs are nearer 

 to each other than either is to any other distinctly recognized 

 epoch in the history of this region. There is, however, still rea- 

 son for considering them as not necessarily identical. Such 

 fauna! evidence as is available does not by any means indicate 

 contemporaneity, and the physical evidence of contemporaneous 

 deposition is far from definite. It seems probable that additional 



