Crosby.] 506 [March 7, 



Carpenter, of the Dakota School of Mines, who kindly introduced 

 me to some of the key sections in the geological structure of the 

 Hills, arrived independently at the conclusion that the Paleozoic 

 series here must be essentially unbroken. 



Relations of the Paleozoic and Mesozoic Formations. 



I had hoped, on entering the Hills, that my observations might 

 contribute to the determination of the upper limit of the Carbonif- 

 erous series. It is an unquestionable fact that nature has drawn the 

 line between the Paleozoic and Mesozoic much less strongly in the 

 west than in the east. As Newton has pointed out, the deposi- 

 tion was, apparently, nearly if not quite continuous in the Rocky 

 Mountain region from the beginning of Paleozoic time to the end 

 of Mesozoic time. Hence, in the almost complete absence of pale- 

 ontological indications, it is not surprising that geologists have 

 failed to agree upon a boundary line. 



Above the variegated sandstone (the Minne Lusa sandstone of 

 Winchell), which Newton places at the top of the Carboniferous, 

 there follow the Red Beds, which include, in ascending order, 75 

 to 100 feet of red arenaceous clay, 15 to 30 feet of purplish, im- 

 pure limestone and 200 to 250 feet of red clay with important 

 deposits of gypsum. The upper or gypsiferous Red Beds, espec- 

 ially, form the floor of the great encircling Red Valley, which is 

 walled on the side towards the hills by the purple and Carbonifer- 

 ous limestones, and on the outside, or toward the plains, by the 

 hard Dakota sandstone at the base of the Cretaceous. 



Although destitute of organic remains, the upper Red Beds are 

 generally conceded to be Triassic, mainly because they are over- 

 lain conformably by beds containing characteristic Jurassic fossils. 

 Winchell believed, however, that he detected an unconformity 

 between the upper Red Beds and the purple limestone ; and he 

 consequently referred this limestone and the lower Red Beds to 

 the Carboniferous. My attention was thus directed particularly 

 to this supposed unconformity, the existence of which is denied 

 by Newton. The exposures of this contact are, however, so un- 

 satisfactory that I failed to find any decisive evidence ; although 

 in the vicinity of Spring Creek and of Whitewood Creek the ap- 

 pearances are certainly not unfavorable to the view that an uncon- 

 formity exists. 



Newton states that, although the close of the Jurassic age wit- 



