3fesozoic and Ccunozoic Geology and FaUuontology. 129 



Dysganus encaustua, D. haydenanus, D. bicarinatus, B. peiganus^ 

 Dicloniiis pentagonus, D. perangulatus, I), calamarius, Monoclonius 

 crassua, Paronychodon lacustris, (Jompsemysimbricarius, C. variolosus, 

 Polythorax missovriensis^ Hedronchns sternbergi, Ceratodus eruci- 

 fervs, C. hieroglyphuSy Myledaphus bipartitus, Loilajjs hazenanus, L. 

 Icevifrons, ZapsaUs abradens, Champsosaurus profundus, C. annectens, 

 O. brevicolUs^ (J. vaccinsulensis, Scaplievpeton excisum, 8. favosum, 

 8. laticoUe, 8. tectum, and Hemitrypus jordananus ; and from the 

 Fox Hills Group, of Montana, Uronautus cetiformis. 



Prof. 0. C. Marsh* described, from the upper Cretaceous of Western 

 Kansas, Ichthyornis victor, HesperornU gracilis, Lestornis crassipes, 

 Pteranodon comptus, P. ingens, P. loiigiceps, P. occidentalism P. velox, 

 and P. gracilis, now Nyctosaitrus gracilis. 



In 1877, Arnold Haguef estimated the thickness of the Cretaceous 

 on the outlying ridges and foot-hills, east of the Colorado range, as 

 follows: Dakota Group* 300 feet; Colorado Group, 1,000 feet; Fox 

 Hills Group, 1,500 feet; and Laramie Group, 1,500 feet. 



The Dakota beds are essentially a sandstone formation, and as they 

 are usually hard and compact, frequently almost a quartzite, the}^ form 

 a well-defined horizon. Ljing between the easilj^-eroded Jurassic 

 marls and clays below, and the overlying blue shales, clays and crumb- 

 ling rocks of the Colorado Group above, the Dakota beds are usually a 

 conspicuous feature in the ridges, which form the foot-hills of the main 

 range. In approaching the mountains from the Great Plains, the 

 Dakota beds are especially prominent, as they form the outlying mem- 

 ber of the series of upturned sedimentarj' beds, which rise so abruptly 

 above the plain; for although the overlying Colorado group is perfectly 

 conformable, they never occur high up on the long ridges, which form 

 a sort of barrier between the level country and the mountain region 

 beyond. 



The Colorado Group is used to represent the Fort Benton, Niobrara, 

 and Fort Pierre Groups. The Fort Benton Group is only exposed along 

 the base of the abrupt ridges, and consists of dark, plastic clays, at 

 times distinctly bedded, and frequently occurring as , thinly-laminated 

 paper shales. . The lower beds are always more or less arenaceous, with 

 iuterstratified'beds of purer clay, while the upper beds sometimes carrj- 

 thin seams of argillaceous limestone, which, in man}^ places, can not be 

 distiuouished from similar beds in the Niobrara. Along the Laramie 



'•■ Am. Jour. Sci. and Arts, 3d Ser., vol. si. 

 t Geo. Sur. 40th parallel. 



