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 cor- 

 related with the divisions of Colorado and Wyoming. 



The whole Cretaceous system of the High Plateaus and their encir- 

 cling 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. Car- 

 bonaceous 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 ajipear 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 Juras- 

 sic, and the Cx-etaceous 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- 



