NO SUBDIVISION OK .M IJA. ] 53 



covered by a broml talus, and so tar as examined it was not foimd to 1)e 

 fossil iferous. 



The relations of the Jura to the other formations of the Hills and 

 the ])lace it occupies in their ph^-sical structure are indicated in the figures 

 that have been g-iven in the previous section to illustrate the structure of 

 the Red Beds and the Red Valley. 



Though it was examined in many places and with considerable care, 

 it was found impossible to base any subdivision of the formation either on 

 persistent lithological characters or on the distribution of fossil forms. 

 There is no single bed that was recognized throughout the belt of expo- 

 sure, and for the most part it was impracticable to correlate, bed by bed, 

 any two sections. 



But while the Jurassic strata vary indefinitely among themselves, they 

 are always easily distinguished from the Red Beds. The change from the 

 deep red clays below to the pale gray clays above is always abrupt, and 

 the latter have a marl 3^ appearance never seen in the former. The heavy 

 gypsum strata of the Red Beds are not found above, although the mineral 

 is not entirely unrepresented. In a few places thin seams of a beautiful 

 flesh-colored fibrous gypsum were found associated with thin limestone 

 strata near the base of the formation. 



The most marked single bed is a sandstone about 50 feet thick, usually 

 of a yellow color, but sometimes quite red. On the Belle Fourche this is 

 about 100 feet above the toj) of the red clays, and forms the floor of a rude 

 platform or slope on the borders of the river, extending backward to the 

 foot of the Dakota cliffs, which at base are formed of Jurassic strata and 

 at top by the Cretaceous sandstone. Its area may be seen by referring to 

 the geological map, for it underlies substantially all that part of the Belle 

 Fourche Valley colored to represent the Jura. A similar stratum on the 

 Redwater is probably equivalent to this; it is separated by 50 or fiO feet of 

 yellowish shale from the red clays below. It is possible, also, that the 

 same is represented by a thinner sandstone seen on the Beaver, 60 or 70 

 feet above the red clays; but in the remainder of the visited exposures it 

 was not recognized. 



It is usual to find this sandstone and others of the formation beauti- 



