OCR Text |
Show 146 GEOLOGY OF 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 usually 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 impossible 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 Colorado 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 formation, 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, |