246 DESCRIPTIVE GEOLOGY. 



through b J a labyrinthine system of dry water-courses, at the head of Cot- 

 tonwood Creek. From the beds exposed in this region, and along the northern 

 borders of the valleys of Henry's Fork, have been obtained most of the re- 

 mains of Eocene vertebrates for which this basin is renowned. A fresh 

 undecomposed specimen of one of the peculiar green sandstones from these 

 vertebrate beds is of a light bluish-green color, and consists of a mixture of 

 fine grains of quartz and black mica with some decomposed feldspar, with 

 a cement of greenish clay. Its analysis, made by Mr. W. R. Woodward, 

 gives the following results : 



Silica - - . - -.--■- 66.17 66.42 



Alumina 14.95 14.73 



Ferric oxide .- 2.76 2.82 



Ferrous oxide — 1.95 1.93 



Manganese - — trace trace 



Lime - - 3.87 3.89 



Magnesia : 1.88 1.97 



Soda --- 2.84 2.97 



Potassa 3.77 3.61 



Lithia trace trace 



Water 2.6J 2.57 



Sulphuric acid trace trace 



100.80 100.91 



The greatest thickness of continuous beds of the Bridger Eocene is 

 found in the face of the bluffs which wall in the valley of Henry's Fork on 

 the north. The higher portions of these ridges are still covered by deposits 

 of Wyoming Conglomerate in an apparently conformable position, and in 

 these places it is probable that the Bridger beds have suffered compara- 

 tively little erosion. On the plateau above Turtle Bluffs to the north, a 

 thickness of some 200 feet of Wyoming Conglomerate still remains, com- 

 posed of coarse gravel and boulders of quartzite, with a small proportion of 

 limestone fragments, in a somewhat calcareous cement. At the Twin Buttes 

 also, where the Bridger beds have an inclination of 2° to the north and west, 

 a thickness of about 40 or 50 feet of the Wyoming Conglomerate caps the 



