88 DESCmPTIVB GEOLOGY. 



The presence of iron in the clays frequently aifects its physical appearance 

 to such an extent as to suggest, when in doubt, to which group the clays 

 may belong. In many localities, the lower clays are quite rich in iron, 

 which forms marked layers or deposits of clay ironstone, cropping out for 

 long distances. At Rock Creek, these clay ironstones are well developed, 

 presenting a characteristic feature of the bluffs, which, owing to the great 

 variety of color derived from the different degrees of oxidation of the iron, 

 and its admixture with clay, gives, on the surface, the appearance of a vol- 

 canic formation of ash and scoria. 



The iron deposits lie with the strata, and are found approximately 

 horizontal, but with a thickness varying from a few inches up to 2 or 3 

 feet. In composition, they are argillaceous carbonates of a brownish-black 

 color and compact texture, which, upon being treated with acid, effervesce 

 briskly, leaving a residue of sand and particles of fine clay. They fre- 

 quently appear considerably fissured, with the seams and cracks filled with 

 crystallized spathic iron. They bear a close resemblance to other beds 

 found in the same horizon on the Wyoming Plains, but especially to the 

 deposits at Elk Mountain, where they are largely developed, and but for 

 the fact that at the latter locality they are turned up at a high angle, show 

 the same mode of occurrence. An analysis of the clay ironstone from Elk 

 Mountain, which will be found in the description of the Cretaceous forma- 

 tion of that locality, probably represents the general character of the Rock 

 Creek deposits. 



In the dark-colored stratified clays, which overlie the carbonate of iron 

 deposits, were found numerous small fish-scales, so characteristic of the 

 Fort Benton division. 



Beyond the railroad-station, Rock Creek runs through a narrow open- 

 ing in. the Colorado clays, which offers steep, almost impassable walls from 

 the stream-bed to the summit, on the top of which the country presents a 

 dreary parched plain, with scarcely any vegetation to relieve the clay 

 barrens or dull monotony of surface features. The Cretaceous strata, which 

 have been dipping eastward, here begin to show the influence of the dying- 

 out of the Medicine Bow Range, and are seen encircling the northern 

 Archaean flanks, gradually trending off to the westward, with east and west 



