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Show 156 GEOLOGY OF THE HIGH PLATEAUS. of Castle Valley, which swing around the north end of the San Rafael Swell and merge into the broad Upper Cretaceous mesas east of it. The fossils which are found in these shales are of brackish-water habits, and although the species are in many cases new or peculiar to the locality, yet their general facies and generic forms are clearly such as harmonize with this correlation. The mass of the Laramie beds is here very considerable, averaging about 1,800 feet. They contain many Carbonaceous shales, and workable seams of coal have also been observed which apparently lie near the base of the group. Between the summit of the Dakota and the base of the Laramie beds lie from 2,000 to 3,000 feet of sandstones and shales which must represent the middle Cretaceous divisions. These are as yet not subdivided nor correlated with the divisions of Colorado and Wyoming. The whole Cretaceous system of the High Plateaus and their encircling terraces is lignitic, and coal is found at many horizons. Nor does one portion of the series seem to abound in coal more than another. Carbonaceous shales are found along the great escarpments in many localities, and a considerable number of workable beds of coal are also known. At the close of the Laramie period we come to a physical break in the course of the deposition. Prior to that epoch the disturbances and resulting unconformities appear to have been few and inconsiderable. The continuity of deposition from the base of the Trias to the summit of the Cretaceous appears to have been unbroken, and the only unconformities seen are local and usually slight. But at the*close of the Laramie period extensive disturbances took place along the old Mesozoic shore line which now marks the boundary of the Great Basin. Considerable areas have been found from which the Cretaceous strata were extensively denuded before the deposition of the earliest Tertiary beds began, and where the lower Eocene is seen to lie across the upturned and beveled edges of the Cretaceous. In the locality now occupied by the Aquarius Plateau and Thousand Lake Mountain the lower Eocene rests directly upon the Jurassic, and the Cretaceous series is wholly wanting over a large part of the area. A great monoclinal flexure runs under the Aquarius from the south, and where it disappears beneath the great lava cap of that plateau the his- |
