150 DESCRIPTIVE GEOLOGY. 



Creek, the Colorado Cretaceous is well represented in a series of three or 

 four low rolls, rising gently above the surface. The strata bend slightly 

 in long gentle S-curves, with a general strike of north 65° east, and dip 85° 

 east. 



Overlying the Dakota, the Fort Benton black shales and bands, with 

 brown earthy marls, appear quite prominently, the latter weathering light 

 gray, with a peculiar dotted, speckled surface. In the marls occurs an 

 undetermined species of a small Inoceramus. The Niobrara light-colored 

 marls, although very characteristic, forming a low continuous ridge of 

 purplish and yellowish beds, would appear to be hardly more than 100 

 feet in thickness, passing up into the overlying clays. North of Sheep 

 Butte, along Rattlesnake Creek, the Colorado Cretaceous, which forms the 

 creek-bed, may be well studied. Here the two lower members of the 

 series crop out upon the south side of the stream, and the Fort Pierre 

 sandy clays on the opposite side pass up into well-defined Fox Hill sand- 

 stones. These upper clays dip 55° to the northeast. 



East of Sheep Butte and south of the Rattlesnake road, the Fort 

 Benton clays are characterized by an extensive development of the fer- 

 ruginous beds, which, in many localities, mark the underlying clay strata 

 in thin bands and nodular concretions. The clays have undergone a very 

 considerable erosion, and are cut by narrow ravines and gullies, exposing 

 the harder iron layers along the ridges for several hundred feet in length, 

 but with a very varying thickness, and with interstratified beds of clays. 

 The iron occurs both massive and in concretionary nodules, associated 

 with rather striking and delicately-marked clay concretions. It presents 

 a steel-black color, weathering to brownish-black, a very even crystalline 

 texture, a conchoidal fracture, and a hardness about 4. In the cracks and 

 fissures and between the surfaces of the nodules occurs a secondary for- 

 mation, caused by percolating waters, of crystalline spathic iron, and an 

 occasional seam of carbonate of lime. The following analysis, made by 

 Mr. B. E. Brewster, shows it to be an argillaceous siliceous carbonate of 

 iron, with a trace of carbonaceous matter, and, like many of the English 

 clay ironstones, containing a considerable amount of manganous oxide. 



