CHAPTER IV. 



GEOLOGICAL FORMATIONS OF THE GRAND RIVER 



DISTRICT. 



In this chapter I shall endeavor to present in as concise a manner as 

 possible the various facts observed in regard to each formation. 



The following list represents those coming nnder observation during 

 the season: 



Upper Carboniferous. 



Triassic ? 



Jurassic. 



^ Lower Dakota group 



Cretaceous } ^PP^' V'^^''^'' ^'''''^'^ 



I Colorado group. 



I Fox Hills group. 



Post-Cretaceous Laramie group. 



^ , . ( Wahsatch group. 



-^^^"^^^ \ Green River group. 



Post-Tertiary and recent. 



On the south side of Grand River (in Area B) there are a few isolated 

 exposures of granitic rock. These were not visited this season, but were 

 referred to in the report for 1875 (p. 66). Neither Silurian nor Devonian 

 was recognized in either district. 



Area A is composed almost entirely of Cretaceous strata, while Area 

 B is about equally divided between the Cretaceous and the Tertiary. 



UPPER CAEBONIFEROUS. 



Area A. — A reference to the accompanying map (Plate YIII) will show 

 the areas in which rocks of the Upper Carboniferous formation are ex- 

 posed. The line separating them from theTrias has been arbitrarily fixed, 

 as the soft sandstones and gypsiferous shales are much like the beds 

 forming the base of the Trias. This is what would naturally be expected 

 when it is remembered that the deposition of the sediments in this region 

 was probably continuous, and the materials entering into their compo- 

 sition derived from the same sources. Both the localities (in Paradox 

 Valley and in the Caiion of the Dolores) noted on the map were seen 

 Irom a distance, and therefore no details respecting them can be pre- 

 sented. They are probably similar to the rocks of the same age showing 

 farther north and west, which were described in the Report for 1875 

 (p. 71). 



Area B. — The map of this area shows a small outcrop of Upper Car- 

 boniferous on Grand River below the mouth of the Rio Dolores. It is 

 unimportant, and was referred to in the Report for 1875. 



JUKA-TKIAS. 



The line separating the Triassic from the Jurassic is almost as obscure 

 as that separating the Carboniferous from the Triassic. The line has 



