430 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



exists among geologists. I, however, succeeded in procuring a number 

 of fossil vertebrates from it, which not only prove conclusively its Me- 

 sozoic age, but its horizontal identity with the reptile-bearing Fort Union 

 beds of the Upper Missouri. This formation, which has been usually 

 regarded as Tertiary, I determined to be Cretaceous in 1869, and the 

 present discoveries establish that view as correct. The fossils which 

 are described in the following pages represent Dinosauria of three spe- 

 cies, a crocodile, and several tortoises, identical specifically with those 

 obtained by Dr. Hayden on the Missouri, Big Horn Elvers, &c. Some 

 of the shells I submitted to Mr. Conrad, and he pronounces them to be 

 Cyrenas. 



South of the South Fork of the Platte, the Cretaceous beds have an 

 extensive development, and south of the Kansas Pacific Eailroad contain 

 some beds of pretty good coal. The high tract of land which extends east 

 from the Eocky Mountains, and constitutes the " divide" between the 

 waters of the Platte and Arkansas, is composed of Tertiary strata lying 

 nearly horizontal. A few days' exploration among them revealed chiefly 

 hard, coarse sandstones and conglomerates, which belong to the Monu- 

 ment Creek group of Hayden. The more elevated hills nearest the 

 mountains are capped by a light-colored trachytic rock, believed to be 

 of volcanic origin. While it overlies the Monument Creek formation, the 

 sandstone of the latter not infrequently incloses angular fragments of 

 a similar rock, showing that the outflow commenced prior to the period 

 of its deposit, and continued subsequently. The age of the Monument 

 Creek formation in relation to the other Tertiaries not having been defi- 

 nitely determined, I sought for vertebrate fossils. The most character- 

 istic one which I procured was the hind leg and foot of an Artiodactyle 

 of the Oreodon type, which indicated conclusively that the formation is 

 newer than the Eocene. From the same neighborhood and stratum, as 

 I have every reason for believing, the fragment of the Megaceratops co- 

 loradoensis was obtained. This fossil is equally conclusive against the 

 Pliocene age of the formation, so that it may be referred to the Miocene 

 until further discoveries enable us to be more exact. 



Fresh- water striita of probable Eocene age were, however, detected 

 by both Dr. Hayden's party and my own in the South Park. These 

 consist of laminated argillaceous shales of soft consistency, in which 

 great numbers of fishes and plant impressions are preserved. The fishes 

 are referable to only two species, Amyzon commune and BMneastes pec- 

 tinatus, and are described in chapter II. They are nearly related to 

 species of the Elko shales and Bridger formation, and I suspect that 

 their age is Eocene. 



From Trout Creek, near Fairplay, we procured a number of inverte- 

 brate fossils of Lower Cretaceous age, a few of which are described by 

 Mr. Conrad in chapter II. 



Thus it appears that, in Colorado as in Dakota, the formations 

 of the Loup Fork, White Eiver, and Fort Union epochs are present, and 

 display a similar succession of life, and that the corresponding horizons 

 display identity in the generic and often specific forms of life. They also 

 exhibit the same marked faunal distinctness from each other in Colorado 

 as in Dakota, and the Colorado fauna displays the same strong diver- 

 sity from the Eocene fauna of Wyoming in respect to the genera, fami- 

 lies, and orders which can be compaTed. 



