Mesozoic and Ccenozoic Geology and Paloiontology. 169 



Ing in other localities to the northward. The Fox Hills group contains 

 a great abundance of well marked Cretaceous fossils, many of the spe- 

 cies identical with those found on the Missouri river. This group passes 

 up into the lignite strata, apparently, without any marked unconform- 

 ability. In passing upward in the Fox Hills Group, one by one the mol- 

 lusca of purely marine character disappear until onl}^ some varieties of 

 oysters remain, with the plants peculiar to the Lignitic Group. 



The relation of the well-defined Cretaceous with the Lignitic Group 

 forms one of the most important problems in Western geology, and 

 the area for the solution of the question probably lies in the Laramie 

 plains and westward toward Salt Lake, where the aggregate thickness 

 is from 10,000 to 20,000 feet. So far, the evidence from the vegetable 

 remains is wholly in favor of the Tertiary age of the coal group. The 

 vertebrate remains, according to Prof. Cope, place the coal group with 

 the Cretaceous, while the proof from the invertebrate fossils is not 

 strong in any direction, although, perhaps, leaning toward the Terti- 

 ary. We must admit, however, that the lower coal-beds are of 

 Cretaceous age so far as the evidence goes. For instance, the Coalville 

 and Bear river beds are most probably Cretaceous, inasmuch as many 

 undoubted Cretaceous t3'pes are found in strata above the coal, and 

 further south, in New Mexico, Arizona, and Utah, there are coal-beds 

 of undoubted Cretaceous age. 



A. R. Marvine* described the Dakota Group between the Big 

 Thompson and South Platte. It can be traced from one point to the 

 other, though it is somewhat obscured near Golden City ; this is due 

 to the fact, that its hardness is greater than the beds either above or 

 below, and it forms a more persistent hog-back ridge than any other 

 group. Between the cross-cutting streams for all this distance and 

 beyond, it rises in its long characteristic ridge, capping t?ie soft Juras- 

 sic beds below, and whether the dip be high or low, usuall}^ reaching to 

 about the same level. The sandstones are usually clean, gritty, even- 

 grained and silicious in texture, var3'ing from a silicious conglomerate, 

 on the one hand, to a hard quartzite on the other, and only occasion- 

 ally becoming soft. Their color is usually light yellow or light gray, 

 or even white, vaiying to rusty yellow, and onh" occasionally red in the 

 softer portions. These are the hard and massive portions which 

 characterize the group, and which are separated by thin, shaly layers, 

 which may be quite argillaceous or even carbonaceous in character, 

 with man}' broken remains of fossil plants. A section at Bear Canon 



- Hayden's U. S. Geo. Sur. Terr. 



