GEOLOGICAL REPORT. 277 



lost all its soluble parts, gypsum, &c, I then found 30 per cent, of alumina, 51 to 55 

 per cent, of silica, traces of calcia, and much water, which was retained with great 

 force, even when the mineral was heated. 



Nos. 8 and 9 then probably correspond to the lowest portion of No. 1 of the Mack 

 Hill section, while No. 10 is lithologically similar to Dr. Hay den's A, the highest bed 

 which he considers as Jurassic. 



On the other hand there are so many local upheavals in the neighborhood of our 

 section that it is not necessary to consider the strata as altogether conformable. Our 

 No. 1, corresponding to No. Ill of the Nebraska Cretaceous section, may be an outlier; 

 it has not been observed anywhere lower down on the river. Nos. 3 and 4 may rep- 

 resent the sandstone formation with lignites of the Upper Cretaceous age, of Nos. IV or 

 V of the Nebraska section, which is most characteristically developed farther south on 

 the North Platte, and of which more will be said below. That no beds of coal have 

 been seen cropping out is no proof against their existence, and, besides, the beds of coal 

 are not uniformly distributed throughout the whole thickness of that formation. Nos. 

 5, 6, and 7 are precisely like some strata which I had, in 1K5<>, observed near the 

 Medicine Bow Butte, resting there upon a gritty limestone which resembles closely 

 our No. 10, but is characterized by its fossils as an equivalent of tfo. II of the Cre- 

 taceous section. They then may form part of No. II or of III. That they are altered, 

 while the other portion of No. Ill, several miles lower down the river, is not altered, 

 would not be a sufficient evidence against their common age. and the apparent absence 

 of fossils in 5 and 7 is only the result of the metamorphie agencies. No. 5 may. how- 

 ever, occupy a lower horizon than No. 1. No. 8 may be an equivalent of Nos. I or II 

 the Cretaceous section, while with 9, probably, the Jurassic formation begins. A thor- 

 ough investigation on the spot is required before the question of the relative age of all 

 w these strata can be settled. 



In Captain Stansbmy's report it is stated that a few miles west of La Bonte Creek, 

 north of Laramie Peak, gray sandstone was seen cropping out, overlying the red sand- 

 stone which we refer to the Triassic age. Above these were layers of red and light- 

 colored shales, impure limestone, and shaly and thinly laminated sandstone, with some 

 Bruchioimla, Monotis, &c. These strata were considered as probably Devonian. Be- 

 sides the fact that the genus Monotis is not known to range so low down, it will be 

 seen from the following that the red sandstone spoken of underlies our Jurassic strata, 

 and that the fossiliferous beds are on a parallel with, or at least closely allied to, our 

 No. 10. They also present the same lithological character, not met with in the more 

 recent rocks of this neighborhood, and are therefore probably of Jurassic age. In the 

 mountains south of the Three Crossings of Sweetwater River, I noticed rocks which 

 are petrographically similar to some of the Jurassic beds, and may be of the same age. 



Either immediately below No. 11 of the above section, or after a repetition of sim- 

 ilar shales and calcareous laminated sandstones of no considerable thickness, the strata 

 near the Red Buttes continue downward in the following order: 



12. Gray sandstone, which I did not examine closely, but noticed only from a 

 distance. It corresponds apparently to No. C of the section of Messrs. Meek and 

 Hayden. It occupies the top of the principal of the Red Buttes, probably, together 



