OCR Text |
Show CBETACEOUS SYSTEM. 155 what part of the Cretaceous system a particular exposure should be assigned, we are rarely in doubt about its Cretaceous age, for each member of the system possesses lithological characteristics only a little less emphatic and distinctive than those of the Trias and Jura. They consist of very heavy alternating masses of iron-gray argillaceous shales and bright yellowish-brown sandstones, which the observer will seldom be in danger of confounding with the members of any other group. The iron-gray shale sometimes gradual^ passes into a bluish-gray or light dove-color, especially to the eastward of the High Plateaus. At the base, or near the base of the Cretaceous system, is a conglomerate, the age of which is doubtful, since the horizon separating the Upper Jurassic has not yet been accurately determined, and the conglomerate may ultimately prove to be a part of the latter group. The upper and lower divisions of the Cretaceous can be correlated with a very high degree of probability with the Laramie and Dakota groups of Colorado, respectively. Our inability hitherto to subdivide the intervening members prevents us for the present from asserting any exact correlations with the middle Cretaceous divisions of that State. The sandstone near the base of the system, with a few underlying shales, is without much doubt the extension of similar strata found in Southwestern Colorado and Northwestern New Mexico by Messrs. Holmes and Peale, and referred by them to the Dakota Group. The fossils found in this group are Ostrea prudentia (White), Gryphea PitcJieri, Exogyra laeviuscala, E. ponderosa, Pli-catula liydrotliecM (White), Avicula linguiformis (Shumard), Camptonectes pla-tessa (White), Callista Beioeyi (Meek and Hayden). In these lower Cretaceous beds are also found a good workable seam of coal and numerous Carbonaceous shales. The coal outcrops near Upper Kanab, south of the Paunsagunt Plateau, and also in Potato Valley, south of the Aquarius.* The equivalence of the Upper Cretaceous shales with the Laramie beds is founded upon their known continuity with strata of that age in Western Colorado and along the course of the Green River south of the Uintas. This continuity can be traced very clearly in the great cliffs west *A good workable coal is found at several places on the southwest flank of the MarMgunt, but I am not quite sure that it belongs to this horizon. |