Crosby.] 510 [March 7, 



appear to indicate a subsidence at the beginning of this age, the 

 lacustrine being, perhaps, largely replaced by oceanic conditions. 



The uplifting of the Black Hills was not, of course, throughout 

 simply a continental movement ; for the marked tilting and plica- 

 tion of the strata on all sides, as we approach the Hills, prove a 

 differential movement, the Hills rising with the continent, but 

 more rapidly. That the Hills appeared as a distinct swell or 

 local uplift, at least as early as Jurassic time, is probable, and if 

 we conceive the deepening of the sea in the beginning of this 

 age as taking place while the Hills were slowly rising, we may, 

 perhaps, find in the quiet erosion of the rounded boss formed by the 

 emergence of the Red Beds and the Carboniferous sandstones a 

 suitable source of a portion of the Jurassic sediments, especially 

 of the reddish and yellowish marls and sandstones, which occur 

 chiefly in the upper Jura. A comparison of Newton's observa- 

 tions reveals the important fact that the thickness of the Jurassic 

 series increases rapidly away from the Hills. It is about 200 feet 

 immediately around the Hills, in the outer wall of the Red Val- 

 ley, 400 feet in the Valley of Red Water Creek and 600 feet on 

 the Belle Fourche, some forty miles from the main mass of the 

 Hills. Although other explanations of this fact are perhaps ad- 

 missible, it is certainly more consistent with the theory that dur- 

 ing Jurassic time the Black Hills were slowly emerging and en- 

 croaching on the Jurassic sea, while the floor of adjacent portions 

 of this sea remained stationary or actually subsided ; than with 

 Newton's view, that the sea covered the entire area of the Black 

 Hills during the whole of Jurassic time. That the Jurassic sea 

 in the vicinity of the Black Hills was shallow throughout is 

 proved, as Newton has remarked, by the abundance of ripple- 

 marks in the sandstones of this age. 



Newton considered that the emergence of the Black Hills did 

 not begin until the close of Mesozoic time. This view, however, 

 not only appears inconsistent with the facts cited from the Juras- 

 sic formation ; but it seems impossible to reconcile it with the os- 

 cillations of the crust and the erosion indicated by the unconform- 

 ity between the Jurassic and Cretaceous at Rapid City, and with 

 the conglomeratic character of much of the Dakota sandstone, 

 the basal member of the Cretaceous. These grit and pebble lay- 

 ers of the Dakota group appear to have been derived from the 

 Archaean rocks of the Hills. 



At the close of the Cretaceous age the entire region of the Black 



