PEALE.] GEOLOGY CRETACEOUS NO. 4 UPPER CRETACEOUS. 137 



They are 25 to 30 feet in thickuess, and represent but a portion of the 

 group. 



Under station 79, on the south branch of Smith's Fork of the Gunni- 

 son, there is a greater thickness exposed, some of No. 3 probably show- 

 ing, although I cannot be certain, as I had no opportunity of making a 

 section. These strata are the direct prolongation of those exposed in 

 the open country between the North Fork and the north branch of 

 Smith's Fork of the Gunnison. 



CRETACEOUS NO. 4. 



Unfortunately opportunity did not oifer to study in detail the strata 

 referred to the Fort Pierre group and the Fox Hills beds. I was unable 

 to tell where No. 3 ended or No. 4 began ; the thickness, therefore, 

 could not be given. I estimated the thickness of the series from No. 2 

 to No. 4, inclusive, at from 1,500 to 2,000 feet. The only fossils obtained 

 were specimens of Avicula linguaformis and Inoceramus barahma, from 

 an outcrop of bluish indurated clay in the valley of the Gunnison, near 

 Kahnah Creek, beneath station 58. This layer, I think, belongs to forma- 

 tion No. 4. No. 4 and No. 5 outcrop in all the localities where Nos. 2 

 and 3 are seen, with the exception of station 73, where No. 2 alone is seen 

 beneath the breccia, the other beds having been eroded previous to the 

 deposition of the breccia. The best exposures are seen in the valley of 

 the Gunnison, but even here the softness of the beds renders them readily 

 eroded, so that they are concealed by the debris of the strata, and it is 

 difficult to make a connected detail-section. The strata of No. 4 are 

 largely light gray and bluish argillaceous beds, with sandstones near 

 the top that give a yellowish debris. They pass gradually into tlie sand- 

 stones and shales of No. 5. In No. 5 the arenaceous character prevails. 

 The group next to be described may belong in part to No. 5, or may, 

 perhaps, be an extension of it. Until fossils are found in it, its position 

 is somewhat indefinite, and I have therefore considered it separately. 



UPPER CRETACEOUS. 



I had no opportunity of estimating the entire thickness of the beds 

 included under this group. In the Elk Mountains, where they are seen 

 most extensively, there has been so much disturbance of the strata that 

 it is difficult to obtain connected sections. 



On Coal Creek there is a bluff in the face of which are exposed 1,500 

 feet of light-gray and yellowish sandstones and shales. On the North 

 Fork of the Gunnison, the exposures must be of greater thickness. The 

 upper part of the series, however, is not shown here. The coal occurs 

 in the lower part of the series, as shown on Coal Creek. The sections 

 will best illustrate the character of the beds. In the broad valley ease 

 of station 48, and south of Grand Eiver, are chocolate-colored shales, 

 and greenish and gray sandstones that should probably be referred to 

 the upper part of the series. The following is a partial section made 

 on a small branch of the Grand, below station 19. 



No 17. — Section south of Grand River. 



Base. Thickness. 



Feet. 



1. Coarse soft whitish sandstone $. 3 



2. Greenish sandstone, coarse and soft 4 



3. Hard brownish limestone .. ) 



4. Space covered with a reddish-brown debris, probably underlaid by al- >.. 15 



tematinc sandstones and limestones S 



