15G GEOLOGY OF THE BLACK HILLS. 



The base of the section is probably some little space above the base 

 of the Jurassic series. On tlie opj)()site side of the creek the red clays with 

 their inclosed gypsum occupy the bottom of the valley. 



In several localities in the vicinity some thin seams of a purple fibrous 

 gypsum were seen in the purple clays (2), but it exists in no great quantity 

 and does not appeTir to be a persistent feature. The massive white sand- 

 stone (4) is extremely unlike the ordinary coarse and deeply colored sand- 

 stones of the Cretaceous. It is so very soft that it readily crumbles into 

 a talus of exceedingly white and fine sand and it was not found at all coarse 

 or conglomeratic, as is usual witli the lower Cretaceous sandstones It is 

 to such an extent intermediate in character between the Cretaceous rocks 

 above and the Jurassic below, that its assignment to the upper formation is 

 made with much hesitation. 



In the Belle Fourche Valle}' a similar soft and white sandstone, about 

 50 feet in thickness, containing bands of pebbly conglomerate, was found 

 separated from the overlying undoubted Cretaceous by 5 feet of an impure, 

 dark calcareous shale. 



At Camp Jenney two or three small hills stand in the Red Valley in 

 front of the main range of the foothills. On the inner side of these hills 

 are seen isolated exposures of the Jura, which are interesting by reason of 

 the great abundance of Belemnites in the gray clays, and a recapitulation of 

 the local section is therefore given. 



Cretaceous. 



Feet. 

 3. Sandstone, capping the hill 100 



Jura. 



2. Clays or marls, greenish, grayish, and yellowish, with a streak of red; imper- 

 fectly exposed, and containing near the base great numbers of Belemnites 

 den.sti.H 165 



1. Sandstone, white and shaly exposed . . 5 



Then a grassed and undulating surface to the Red Beds and gypsum 

 below, the base of the Jura being concealed. 



The next examination was made on Red Canon Creek, thirty-five miles 

 south of Camp Jenney. The following section was observed in a small 



