CRETACEOUS 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 con- 

 founding with the members of any other group. The iron-gray shale some- 

 times gradually 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 Cre- 

 taceous system, is a conglomerate, the age of which is doubtful, since the 

 horizon separating the Upper Jurassic has not yet been accurately deter- 

 mined, and the conglomerate may ultimately prove to be a part of the latter 

 group. 



Tiie 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 sand- 

 stone 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), Gn/phea PitcJieri, Exogyra laeviuscula, E. ponderosa, Pli- 

 catula hjdrotheca (White), Avkula lingmformis (Shnmard), Camptonectes pla- 

 tessa (White), CaUista Deweyi (Meek and Hayden). In these lower Creta- 

 ceous 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 jjlaces on the southwest flank of the Mark^gunt, 

 but I am not quite sure that it belongs to this horizon. 



