| OCR Text |
Show SUBSIDENCES. 13 period. In other words, the lake contracted its area from south to north during at least the latter half„ of the Eocene, and at the close of that age finally disappeared. SUBSIDENCE OF CRETACEOUS-EOCENE SEDIMENTS. A most interesting but perplexing problem is suggested when we consider the enormous bulk of the Cretaceous-Eocene strata of the Plateau Province and the peculiar circumstances under which they were deposited. The whole series abounds in coal and carbonaceous shales, and remains of land plants are abundant, even where carbonaceous matter is absent. If current theories of the formation of coal are not radically wrong, we seem compelled to believe that throughout that vast stretch of time which extended from the base of the Cretaceous to the summit of the Eocene the whole province, with the exception of a few possible but unknown land areas, maintained its level almost even with that of the ocean. The Dakota sandstone could not have been deposited here much if any below that level, nor the Wasatch beds much if any above it. And yet we have the paradox that 6,000 to 15,000 feet of strata were deposited over an area of more than 100,000 square miles with comparatively few unconformities arid contemporary disturbances, while the level of the uppermost stratum always remained at sensibly the same geographical horizon! It is incredible that the Cretaceous ocean at the commencement of that age could have had a depth equal to the thickness of the strata and that the sediments filled it up. The facts are wholly against such a supposition, and point clearly to shallow waters. The only conclusion which appears tenable is that the strata sank as rapidly as they were deposited. The case is analogous to that of the Appalachians during Palaeozoic time, and especially during the Carboniferous; and the more we reflect upon the similarity the stronger does it become. It fails, however, when we come to consider the phenomena presented in the two regions in the period subsequent to the deposition; the Appalachian strata were flexed and plicated to an extreme degree, while those of the west are for the most part calm and even. Only in the vicinity of the mountains and shore lines do we find them much disturbed. |
