174 GEOLOGY. 



very long periods. In the Rocky Mountains the Triassic deposits 

 lie in unconformable masses, directly on or against the Archaean 

 islands that form the back-bone of the continent.* Here the Tri- 

 assic forms a series of sandstones from three hundred to one thous- 

 and feet thick, whiph are loose, friable sediments wherever there is 

 an approach to a horizontal position. "On approaching the Arch- 

 aean, the Trias always is composed, or largely made up of con- 

 glomerates, the materials of which were derived, from the shores 

 against which they abut." Clarence King. Towards the eastern 

 part of the Uintas the Trias thicken still more, reaching finally a 

 depth of from two thousand to twenty-five hundred feet.. Still 

 farther westward the Trias diminishes in thickness and increases in 

 compactness and the quantity of conglomerates. From these facts 

 Clarence King concludes that there was a land mass towards the 

 west, during this period from which the materials that enter into its 

 deposits were derived. 



Overlying the upper beds of Triassic rocks, which are intercal- 

 latecl with gypsum and dolomitic limestone, are the Jurassic beds, 

 which are first met in the eastern flank of the Colorado range. 

 Here they are only two hundred and fifty to two hundred and sev- 

 enty-five feet in thickness, and increase westward, until, on the 

 Wasatch, they are eighteen hundred feet thick. The Jurassic is 

 almost entirely made up of soft clays, clayey calcareous marls, and 

 intercalations of fine lithographic limestone. These rocks are 

 therefore a lime and clay deposit. Clarence King. The maximum 

 development of the Triassic and Jurassic, east of the Wasatch, is 

 not less than thirty-eight hundred feet. 



Immediately above the Jurassic, on the eastern foothills, lies a 

 " heavy bed of conglomerate, which is the base member of the 

 Dakota Cretaceous. * * "The upper clay and sandstone beds 

 directly under the bottom of the Dakota conglomerate have been 

 called by Marsh the Atlantasaurus beds." Clarence King. Hay- 

 den and Meek have shown that it is probable that the Jurassic 

 beds extend eastward beneath the Cretaceous. As the Cretaceous 

 extends in turn beneath the Tertiary, it is possible that there may 

 be Jurassic beds in western Nebraska that cannot be observed, ow- 

 ing to the thickness of the overlying deposits of later geological 

 periods. This is the more probable, since during Jurassic times 

 there was a deepening of what had been the old Triassic seas, and a 



*King's Report ou the Fortieth Parallel. 



