Mesozoic and Ccenozoic Geology and Palceonfology. 139 



tinct remains of plants occur, and in the higher shales a Gryphoia. 

 The thickness is about 400 feet. 



2. The Colorado Group, consisting of an extensive series of dark 

 graj', slightly calcareous shales. They are thinly; laminated, easily 

 eroded, and become light ijray or whiteupon exposure. Covering the 

 highest portions of the region l^'ing between Sheep Mountain and the 

 base of the third chain, they present comparatively steep bluffs par- 

 allel to their strike, and rounded surfaces along their dip. A few banks 

 of argillaceous limestone may be found within them. Within the 

 upper third the shales are more arenaceous than lower down. A cold 

 sulphur spring near Camp Brown seems to take its rise in these shales 

 which must be regarded as a ver}- jjrolific source for alkaline com- 

 pounds of a highly soluble nature. Within the shales there are small 

 inclusions of p3'rite. Upon decomposition of this and the shales 

 various salts are formed. The thickness of this group is about 600 

 feet, increasing southerly to 900 feet. 



3. The Fox Hills Group, consisting in the lower part of brown and 

 j^ellow shales, interstratified with thin beds of sandstone. Some of the 

 shales are very dark and carbonaceous. Above this alternating series 

 there is a considerable thickness of yellow and brown shales. As a 

 rule, they are arenaceous, but some of them quite free from sand. 

 Small particles of mica occur throughout. Higher up, sandstones set 

 in again, containing, together with thin seams of shales, small deposits 

 of coal. The upper part is formed \iy thinly-bedded, micaceous and 

 argillaceous sandstones, covered by a thick stratum of the same 

 material. The thickness is estimated at 500 feet. 



About two miles west of Camp Brown, a very interesting hot spring 

 oc6urs, w*^ich rises in the beds of this group. It is known as the Hot 

 Sulphur Spring. The temperature is from about 100° to 110°, and 

 varies but little with the weather. The bright green and blue water is 

 contained within an elliptic basin 315 feet long and 250 feet wide. A 

 constant bubbling up of carbonic-acid gas gives it the appearance of 

 boiling. The mineral constituents held in solution by the water are 

 iron, lime, magnesia, soda and potash. They seem to be contained in 

 the form of sulphates, carbonates and chlorides. The heat which sup- 

 plies the warmth of the water is supposed to be due to chemical 

 changes going on within the strata through which the moisture finds 

 its way. A petroleum spring also occurs near Camp Brown, originating 

 probably in the same rocks. 



The Laramie Group consists of a succession of shales and yellow 



