Article XXXI. — ON JURASSIC STRATIGRAPHY ON 

 THE WEST SIDE OF THE BLACK HILLS — SEC- 

 OND PAPER ON AMERICAN JURASSIC STRATIG- 

 RAPHY. 



By F. B. LooMis. 

 Plates LIV and LV. 



During June, 1901, a party, sent out by Professor Osborn, 

 from the American Museum of Natural History, prospected 

 the Jurassic exposures of the west side of the Black Hills for 

 Dinosaur remains. In connection with this work sections 

 were made by the writer at six of the best and most character- 

 istic exposures, to show the stratigraphy of these deposits. 

 These were made at Professor Osborn's request for com- 

 parison with the sections previously made by the writer in the 

 Como district of Wyoming. 



The area covered extends from a little north of Hulett to 

 fifteen miles south of Newcastle; the exposures stretching 

 some 125 miles in length, and varying from a fourth of a mile 

 to ten or twelve in width. The distribution of the Jurassic 

 in the region is shown in the map accompanying the paper, 

 as is also the location of the individual sections. The Juras- 

 sic is exposed mostly in escarpments, capped by the heavy 

 Dakota sandstone, which makes the 'rim.' These escarp- 

 ments generally face toward the centre of the Hills and con- 

 tinue on around the north and east sides as well as on the 

 west side. The soft clays which predominate in the Upper 

 Jurassic are the cause of a considerable valley all around, just 

 inside the 'rim.' The strata dip in varying degrees away 

 from the centre of the Hills, but are in the best exposures 

 nearly horizontal. 



The Jurassic is divisible into two parts: a lower marine, 

 corresponding to Knight's Shirley'; and an upper fresh or 

 brackish water corresponding to Scott's Como.^ On the east 



^ W. C. Knight, 'Jurassic Rocks of Southeastern Wyoming,' Bull. U. S. Geol. Surv., 

 Vol. XI, pp. 377-388. 



« W. B. Scott, Introduction to Geology, p. 447 (footnote). 



[401] 



