386 N. H. DARTON — JURASSIC FORMATIONS OF THE BLACK HILLS 



The upper limit of the Jura is so placed by Newton in most cases as 

 to comprise only the beds containing Jurassic molluscan remains, but 

 in places it includes a greater or less amount of overlying sandstones. 



Several years ago O. C. Marsh collected saurian remains from the 

 shales overlying the marine Jurassic beds near Piedmont, on the east 

 side of the Hills. He also obtained through a local collector a large 

 number of cycads from the sandstone near Piedmont and other places 

 which was classified as Dakota by Newton. The saurian remains are 

 Jurassic, but Marsh* has been disposed also to regard the cycads as the 

 same in age. 



The cycad-bearing sandstones have been examined by L. F. Ward at 

 several points in the Hills, and W. P. Jenney has collected many plants 

 from coals associated with these sandstones in the northern Hills. It 

 is Ward's t opinion that this flora is Lower Cretaceous. 



In 1898 I discovered saurian bones at or near the cycad horizon, at 

 Buffalo gap, as announced to this society in December of that year. 

 Doctor Lucas, who is studying the bones, is not prepared to give a decided 

 opinion as to whether they are early Cretaceous or late Jurassic, but 

 they would usually be regarded as Jurassic; so it is that the upper 

 limit of the Jurassic in the Black hills is perhaps open to question. 

 This, however, need not be discussed in the present paper, as it will be 

 treated by Doctor Lucas and the writer in a later communication. 



Relations of the Base of the Jurassic 



The base of the Jurassic is clearly defined throughout the area of the 

 Hills by an abrupt change in the character of the sediments, more or 

 less slight erosion, and the evidence of an entirely different histor^^ from 

 that of the underlying Red beds. Some features of the Jura-Trias con." 

 tact are shown in the two figures of plate 44 (page 393). 



Classification, Composition, and Nomenclature of Black Hills 



Formations 



From the result of my recent study it appears desirable to formulate 

 the following classification and nomenclature for the Black Hills Juras- 

 sic and adjoining formations: 



*Am. Jour. Science, 4th series, vol. 0, p. 167. 



+ Journal of Geol., vol. 2, pp. 250-266, and 19tl) Annual Report U. S. Creologieal Survey, Part n, 

 pp. .521-958. 



