326 GEOLOGICAL SUEVEY OF THE TERRITOEIES. 



by bluffs of from 20 to 150 feet in heiglit on its southern side, while the 

 northern rises gradually into the prairie. These bluffs are of yellow 

 chalk, except from ten to forty feet of blue rock at the base, although 

 many of the canons are excavated in the yellow rock exclusively. The 

 bluffs of the upper portion of Butte Creek, Fox and Fossil Spring (five 

 miles south) Caiions are of yellow chalk, and the reports of several 

 persons stated that those of Beaver Creek, eight miles south of Fossil 

 Spring, are exclusively of this material. Those uear the mouth of Beaver 

 Creek, on the Smoky, are of considerable height, and appear, at a dis- 

 tance, to be of the same yellow chalk. 



I found these two strata to be about equally fossiliferous, and am 

 unable to establish any paleontological difference between them. They 

 l)ass into each other by gradations in some places, and occasionally 

 present slight laminar alternations at their line of junction. I have 

 specimens of CimoUchthys se7nianceps, Cope, from both the blue and yel- 

 low beds, and vertebrgg of the Liodon glandiferus, Cope, were found in 

 both. The large fossil of Liodon dyspelor, Cope, was found at the junction 

 of the beds, and the caudal portion was excavated from the blue stratum 

 exclusively. Portions of it were brought east in blocks of this material, 

 and these have become yellow and yellowish on many of the exposed 

 surfaces. The matrix adherent to all the bones has become yellow. A 

 second incomplete specimen, undistinguishable from this species, was 

 taken from the yellow bed. 



As to mineral contents, the yellow stratum is remarkably uniform 

 in its character. The blue shale, on the contrary, frequently contains 

 numerous concretions, and great abundance of thin layers of gypsum 

 and crystals of the same. Near Sheridan, concretions and septaria are 

 abundant. In some places the latter are of great size, and being im- 

 bedded in the stratum have suffered denudation of their contents, and 

 the septa standing out form a huge honey-comb. This region, and the 

 neighborhood of Eagle Tail, Colorado, are noted for the beauty of their 

 gypsum crystals, the first abundantly found in the Cretaceous formation. 

 These are hexagonal-radiate, each division being a pinnate or feather- 

 shaped lamina of twin rows of crystals. The clearness of the mineral- 

 and the regular leaf and feather forms of the crystals give them much 

 beauty. The bones of vertebrate fossils preserved in this bed are often 

 much injured by the gypsum formation which covers their surface, and 

 often penetrates them in every direction. 



The yellow bed of the Niobrara group disappears to the southwest, 

 west, and northwest of Fort Wallace beneath a sandy conglomerate of 

 uncertain age. In color it is light, sometimes white; and the component 

 pebbles are small and mostly of white quartz. The rock weathers irreg- 

 ularly into holes and fissures, and the soil covering it is generally thin and 

 poor. It is readily detached in large masses, which roll down the bluffs. 

 No traces of life were observed in it, but it is probably the eastern mar- 

 gin of the southern extension of the White Eiver Miocene Tertiary 

 stratum. This is at least indicated by Dr. Hayden in his geological 

 preface to Leidy's Extinct Mammals of Dakota and Nebraska. ' 



Economically the beds of the Mobrara formation possess little value 

 except when burned as a fertilizer. The yellow chalk is too soft in many 

 places for buildings of large size, but it will answer well for those of 

 moderate size. It is rather harder at Fort Hays, as I had occasion to 

 observe at their quarry. That quarried at Fort Wallace does not apiDcar 

 to harden by exposure ; the walls of the hospital, noted by Leconte on 

 his visit, remained in 1871 as soft as they were in 1807. A few worth- 

 less beds of bituminous shale were observed in Eastern Colorado. 



