228 RETORT UNITED STATES GEOLOGICAL SURVEY. 



of all ages in this region from those of the Colorado to the Bridger Group, 

 inclusive. As to its unconformity upon the Bridger strata, however, 

 my observations south of the Uinta Mountains alone would not prove, but 

 I found such unconformity a few years ago in the valley of Snake River, 

 north of Junction Mountain. 



Leaving the region of the White and Yampa Rivers I crossed Green 

 River by a ford a few miles below Split Mountain and continued my 

 journey westward after making some observations from that mountain 

 to add to my report for the year 1870. After crossing Green River I 

 spent some time in examining the geology of the district on the Avest 

 side adjacent to the southern base of the Uinta Mountains, especially in 

 the valleys of Brush Creek and Ashley's Fork. In this district, as well 

 as in that which lies immediately upon the other side of Green River, I 

 made some observations that have a most important bearing upon the 

 proper correlation of the different groups of strata which geologists have 

 recognized, but more especially those of Cretaceous age. In a large part 

 of Colorado, Wyoming, and Utah the two Cretaceous groups, which in the 

 classification modified from that which was originally adopted by Hay- 

 den and Meek for the Upper Missouri River region are designated as 

 the Colorado and Fox Hills Groups, have been found so constant in their 

 general lithological characteristics that field geologists have usually 

 made these the basis of their classification of the strata, often ignoring 

 the paleontological features entirely. My own investigations have led 

 me to the conclusion that the paleontological characteristics of these 

 groups are far more constant and reliable than the lithological, and this 

 fact is especially exemplified in the district in question. Generally the 

 plane upon which the Colorado and Fox Hills Groups are separated is 

 marked by a more or less sudden change from a shaly or uncompacted 

 sandy material below to ordinary stratified sandstone above. A large 

 part of the Colorado Group, especially toward its base, is also usually 

 made up of bluish clayey and sandy shales, with usually a horizon of 

 bluish fissile shales at or very near the base of the group. Often, how- 

 ever, the lithological change from the Colorado to the Fox Hills Group 

 is very obscure, the sandy shales of the lower group extending far up 

 into the upper one. 



In the district adjacent to Green River, at the southern base of the 

 Uinta Mountains, more than half the thickness of the Fox Hills Group is 

 inseparable from the Colorado Group by lithological characters, and 

 their separation is thus practicable only by means of their respectively 

 characteristic fossils. It is true that the relative thickness of these two 

 groups varies very considerably in different districts, and this fact is 

 never more plainly or truthfully shown by lithological than by paleonto- 

 logical features. In short there is, as a rule, in all the great western 

 region, a distinctly recognizable paleontological horizon separating the 

 two groups in question, irrespective of lithological variation, above and 

 below which certain species respectively do not pass. For example, on 

 both sides of the Rocky Mountains I have found Inoceramus deformis 

 Meek, I. problematieus Schlotheim, and Ostrea congesta Conrad quite 

 common if not abundant in the Colorado Group, while none of the Creta- 

 ceous species of any of the foregoing lists in this report have been found 

 in that group, but all belong above it.* 



* Iu my report upon the invertebrate fossils of Professor Powell's collections, in chap- 

 ter III of his Geology of the Uinta Mountains, a considerable number of the Cretaceous 

 species there discussed are referred to the Colorado Group ( = Sulphur Creek Group of 

 Powell), which I have now no doubt properly belong to the Fox Hills Group ( = Salt 

 Wells Group of Powell). The error made by the collectors, of referring the fossils to 

 wrong groups, no doubt occurred in consequence of tho lithological changes that have 

 taken place in their strata, which has just been explained. 



