CAlMJONIFEKOrs AND ^lESOZOIC. 43 



impure argillaceous limestone from a similar red clay 200 to 300 feet thick, 

 containing large quantities of pure white gypsum. Assiduous search, especi- 

 ally in tlie limestone, revealed not the slightest trace of organic life in the 

 series, and there is nothing new to present that will aid in settling the ques- 

 tion of the true age of the formation. 



In connection with the occurrence of the Jurassic formation, the 131ack 

 Hills have again a historical interest, for it was first recognized in the geo- 

 logical series of the West from fossils collected here by Dr. Hayden. Its 

 separation from the underlying Red Bed series is tolerably well defined by 

 lithological change, but is especially marked by the occurrence of Jurassic 

 fossils close to the line of division. From the Dakota sandstone of the 

 Cretaceous, whose harder strata everywhere form the overlying and pro- 

 tecting cover to the softer Jurassic beds, the separation of the Jura is less 

 plainly denoted; for the upper portion of the Jura contains much sandstone 

 and is usually unfossiliferous. Fortunately we Avere enabled to make a 

 large collection of Jurassic fossils, among which are many of the species 

 described by Professor Meek, upon which he based his determination of the 

 age of the rocks, besides many new forms. 



The Cretaceous rocks were not studied with the care that was given 

 to the older series, mainly because the opportunities were not so favorable. 

 The lower member, or Dakota sandstone, is a prominent feature in the topog- 

 raphy, forming, as it does, the chief stratum of the rampart foothills, but 

 the separation of the other groups, as established by Meek and Hayden, 

 was not so clearly made out, though they are exposed on all sides of the 

 Hills. The general similarity in the nature of the deposits and the scarcity 

 of good exposures were hinderances to their accurate examination. With 

 some modifications of thickness, the divisions established by Professor 

 Meek, and described by Dr. Hayden in various parts of the Upper Missouri 

 region, are equally applicable in the Black Hills, and to their description 

 there is little to be added or changed as a result of our examinations. Some 

 recompense for the insufficiency of our study of the formation is found in 

 the fine collection of invertebrate fossils which it was our fortune to obtain 

 from it. The perfection, number, and beauty of the specimens can scarcely 

 have been excelled by any other collection from this region. 



