146 GEOLOGY OP THE HIGH PLATEAUS. 



Thoughout the region lying- between the Great Plains of Colorado 

 and Wyoming and the Basin area, wherever the horizons from the summit 

 of the Carboniferous to the base of the Jurassic are exposed, there are usu- 

 ally found sandstones and arenaceous shales, distinguished by their rich red 

 coloring, their tolerably constant texture and appearance, and the absence 

 of fossils of distinctive character. In many places they may be imperfectly 

 resolved into two groups, though ordinarily they show no well-marked 

 plane of division between them ; the distinction being somewhat vague and 

 uncertain. The Triassic age of the upper portion is pretty well ascertained. 

 Mr. Clarence King has found fossils in the lower portion which he believes 

 to be sufficient to justify him in calling it Permo-Carboniferous. But the 

 want of a clear boundary between the two divisions of these "Red-beds" 

 has led many geologists to regard them provisionally as one formation, 

 under the name of Trias. In the Plateau Country these beds appear to 

 be conformable with each other, while the contact with the Carboniferous 

 below is in several places distinctly unconformable. They gradually pass 

 into the Trias above, and if a divisional plane is to be drawn, it is impossi- 

 ble to locate it within a belt of 500 feet of monotonous shales, and hence 

 the tendency has been to regard the whole series as one group, and to use 

 the names Upper and Lower Trias for the designation of different portions 

 which, in reality, are not at present distinctly and precisely separable. 

 Perhaps, also, some hesitation arises from the importance which must attach 

 to a full recognition of the Permian age of these lower beds. The identity 

 of the Shinarump of Utah and Arizona with the lower Red : beds of Colo- 

 rado and Wyoming is unquestionable, and the formation, therefore, covers 

 an area probably exceeding 250,000 square miles, with many exposures, 

 and there is no intrinsic improbability that it is buried beneath a still 

 greater area. If its age be Permian, then the Permian becomes a forma- 

 tion, ranking in importance stratigraphically with the Trias and Jura, and 

 can no longer be considered as a merely local deposit coming in here and 

 there to round off the majestic proportions of the Carboniferous. While the 

 Permian age of these beds, therefore, is quite possible, there is good reason 

 for laying a heavy burden of proof upon the advocates of that view. 



The thickness of the Shinarump formation is difficult to determine, 



