134 GEOLOGY OF THE BLACK DILLS. 



montary structure. It lias, liowever, in places a sandy phase, and in that 

 condition it is a plainly laminated shale or shaly sandstone. In the absence 

 of observable bedding, the thickness had to be determined indirectly by 

 measuring across the outcrop and combining the result with observations on 

 the dip and profile of the adjacent strata above and below. From a num- 

 ber of such estimates a general thickness of 200 or 250 feet was deduced. 



The arenaceous phase of the clay was observed chiefly in the Belle 

 Fourche region near Bear Lodge, in portions of the Redwater Valley, and 

 on lower Red Canon Creek. 



A sample of the material in its typical condition of a fine plastic clay 

 was submitted to Dr. Ricketts for a qualitative analysis and was found to 

 contain "alumina, silica, lime, and iron (as protoxide and peroxide) in quan- 

 tity, with traces of soda, potash, baryta, and carbonic acid. It is essentially 

 a silicate of alumina (clay), with some silicate of iron, containing also some 

 carbonate of lime and colored by sesquioxide of iron." It does not contain 

 sufficient carbonate of lime to warrant the title of marl, but critically it may 

 be considered a clay more or less silicious and colored by peroxide of iron. 



Gypsum is found in intercalated beds in great abundance, but its 

 exposures are not always easily traced The beds have probably great 

 variation in thickness, and their outcrops are apt to be covered by thin 

 layers of clay. They were observed from 2 to 10 feet in thickness, and 

 could probably be found considerably thicker if excavations were made. 

 Our examinations did not suffice to determine whether they are more abun- 

 dant in one portion of the clay than in another, nor to trace for any consid- 

 erable distance a single stratum. They were found from within 75 feet of 

 the purple limestone to the very summit of the well distinguished red clays, 

 lying but a few feet below characteristic Jurassic fossils. 



In man}^ places in the vicinity of the gypsum strata the red clay is 

 found penetrated by a net-work of thin seams of gypsum ranging in width 

 from an inch to the thickness of a sheet of paper, cutting the clay in all 

 directions and interlacing in the most intricate manner. These are unques- 

 tionably a secondary deposition formed by the infiltration in crevices and 

 cracks in the clay of waters bearing in solution gypsum derived from the 

 associated beds. The gypsum where exposed has been largely removed by 



