180 GEOLOGY OF THE BLACK HILLS. 



tion as caps the cliffs southeast of Bear Lodge. The top of the cliff is 950 

 feet above the Belle Fourche. 



Feet. 



4. Sandstone, yellowish with red stains; containing layers of conglomerate 75 



3. Hark gray calcareous sbale and dark impure limestone 5 



2. Sandstone, very soft and white, with layers of fine conglomerate 50 



1. Clays and marls of the Jura — 



Twenty-two miles farther down the river the dip of the sandstone 

 carries it down to the level of the stream, and it disappears beneath the 

 overlying clays of the Fort Benton group. At this point the following 

 section was observed: 



Feet. 



5. Sandstone, shaly 25 



4. Sandstone, reddish and yellowish 100 



3. Shales or clays, gray, yellow, and red 50 



2. Sandstones, white and soft ... 75 



1. Jurassic clays or marls, sandy, gray, red, and yellow — 



Though the Dakota group contains occasional strata of clay or shale 

 there is rarely any difficulty in separating it from the dark plastic clays of 

 the overlying Fort Benton. 



Of the Fort Benton and upper groups of the Cretaceous no general 

 study was made, and the little that can be said is a very slight contribution 

 to their history. They were seen only on a few lines of travel and for the 

 most part under unfavorable circumstances. 



Between Bear Butte and the foothills the undulating country appears 

 to be underlaid entirely by the black clays of the Fort Benton group (No. 

 2), but we found no fossils. East of the butte No. 3 appears, and on the 

 banks of the Belle Fourche near the mouth of Bear Butte Creek are the 

 dark gray clays and shales of the Fort Pierre, No. 4, with a few character- 

 istic shells, Bacidites, Ammonites placenta, etc. 



Southward along the base of the foothills the Fort Benton is easily 

 traced by the fineness and darkness of its clays. They are remarkable for 

 their large proportion of soluble salts, which contaminate all the streams 

 that pass through or rise in them, rendering their waters bitter and medic- 

 inal. In their vicinity it is usual to see a white deposit or crust lining the 

 banks of the streams, and occasionally alum springs are met with. The 



