130 GEOLOGY OF THE BLACK HILLS. 



age, and indeed half of the fragments that one picks np betray in some 

 way an organic origin, Init the preservation is so poor that it is only here 

 and there that a form can be distinguished. Our collections add but four 

 forms to the list of those gathered by Professor Winchell and Mr. Grinnell 

 during the Custer expedition of 1874. The forms thus far obtained from 

 the lower member (1) are — 



Zaphrentis centralis f Spirifera. 



Campophyllmn (or AmpJexus). Productus. 

 There have been brought from the middle members (2 and 3): 



Zaphrentis. Terehratida. 



Syr'wgopora muUattennata. Spirifera Eochj-montana. 



Atlnjris suUilita. Retzia. 



Streptorhjnclms. Euomplialus. 



Rhynclionella. 

 On the eastern coast of the United States the Carboniferous period was 

 followed by an epoch of great disturbance, in which the strata of the Car- 

 boniferous and the preceding periods were bent, twisted, and plicated in a 

 grand manner, and the Alleghany Mountains were produced. During this 

 disturbance there was also an extinction of old organic types and an usher- 

 ing in of new ones. The Mesozoic strata deposited subsequently are, there- 

 fore, found resting upon or abutting against the older formations uncon- 

 formably. In the Far West, too, in the region of the eastern flanks of the 

 Rocky Mountains, observers have found a similar unconformity between 

 the Paleozoic and Mesozoic. 



In the Black Hills, however, we were able to detect no evidence of un- 

 conformity between the Carboniferous and the overlying Red Beds. There 

 is even such a lithological gradation from Paleozoic to Mesozoic that in the 

 absence of fossils the line between the two cannot bo drawn with confidence. 

 It is true, indeed, that the length of outero]) continuously exposed was not 

 of sufficient extent to betray a slight discordance of dip, but from the com- 

 parative study of the strata at man}' different localities no evidence was 

 obtained that would warrant us in assuming any want of conformity. For 

 aught we know the area now included in the Black Hills was the scene of 

 continuous deposition from the beginning of Paleozoic time to the end of 

 Mesozoic time. 



