• TEETIAEY PLAINS OF WYOMING. 71 



thin lenticular seams of a jasper-like rock. This rock is traversed by nar- 

 row lines of calcareous spar and silica. It possesses a conchoidal fracture, 

 a homogeneous texture, and in color is always some light shade of gray, 

 green, or brown*. In many localities, it is penetrated by dendritic forms of 

 iron or manganese. Moss-agates are found scattered through the marls in 

 small fragments and chips, which have been eagerly searched for by the 

 inhabitants. In one or two localities, they occur so abundantly as to form 

 an article of commerce. They were purchased by a firm of manufacturing 

 jewellers, in Cheyenne, at a fixed price per pound, from which they obtained 

 large numbers sufiiciently clear and perfect to polish for ornamental stones. 

 Most of the moss-agates which we see worn are obtained from the Tertiary 

 plains of Wyoming. Many of them show a great variety of colors, with 

 beautiful opalescent tints, and are frequently very rich in the enclosed den- 

 dritic forms of iron and manganese. Fine specimens have been found on 

 the Chugwater, near the crossing, and again on Horse Creek, 12 miles east 

 of the Military Road. 



Silicified wood is also scattered through the Pliocene basin, but is a 

 less marked feature than in the Tertiary beds of Nevada. 



As regards the life of the Niobrara basin, invertebrate remains would 

 appear to be rare; at least, none were found by our parties. But of verte- 

 brate fossils, Prof. 0. C. Marsh has made large collections from numerous 

 localities, especially from the upper beds at Chalk Bluffs, many of the 

 species being identical with those found in the same formation from the 

 "Mauvaises Torres". 



Among the more important species obtained by Professor Marsh may 

 be mentioned the following: 



Mastodon murificus, Leidy. 

 ElepJias imperator, L. 

 ProfoJiippus parvulus, Marsh. 

 Pliohippus, Ilipparion. 

 These forms show a warm temperate climate 



Wyoming Conglomerate. — Clearly overlying the Pliocfene lake- 

 deposits are found, along the base of the range, beds of both coarse and 

 fine conglomerate, which hold a somewhat doubtful geological position. 



