42 DESCEIPTIVE GEOLOGY. 



presence is very easily recognized, and in many localities either one or the 

 other bed occurs well developed. 



The Fort Benton beds, which lie next the Dakota, and are only exposed 

 along- the base of the abrupt ridges, consist of dark plastic clays, at times 

 distinctly bedded, and frequently occurring as thinly-laminated paper-shales. 

 The lower beds are always more or less arenaceous, with inters^tratified beds 

 of purer clay, while the upper beds sometimes carry thin seams of argilla- 

 ceous limestone, which, in many places, cannot be distinguished from similar 

 beds in the Niobrara. In general, the beds are very uniform in lithological 

 character. Along the Laramie Hills, they are somewhat difficult to recog- 

 nize, as they are usually concealed by loose soil and debris from the steeper 

 ridges to the westward, but in Colorado they may be traced for long dis- 

 tances in well-defined north and south lines. 



The Niobrara beds, which occur just above the Fort Benton, although 

 much thinner, are, from their characteristic colors and peculiar mode of 

 weathering, more easily recognized. Where the upper beds of the Fort 

 Benton are calcareous, it is generally difficult to separate the two series 

 below the yellowish-white chalky limestone strata, which is so pronounced 

 a feature in the Niobrara. They consist mainly of marls and impure 

 limestone. The lower members are bluish-gray argillaceous limestones, 

 with occasional beds of lighter-colored clays, all of them highly fos- 

 siliferous, passing up into yellowish- white or cream-colored marls, associated 

 with gypsum, which occurs frequently in thin, irregular layers, and at times 

 disseminated through the marls. Not far above this occurs a second bed of 

 somewhat deeper yellow marl, which at the top passes into bluish-gray, soft, 

 earthy beds. 



The base of the Fort Pierre group is usually much obscured by over- 

 lying soil, and at times the lithological aspect of the beds blends so com- 

 pletely with those of the underlying Niobrara that any separation seems 

 impossible ; but in other localities the dark, almost black, thinly-laminated 

 clay affords a very distinct horizon. The entire group, like the Fort Benton 

 division, is made up of clay-beds, with but little variety in general character. 

 The lower clays carry considerable amounts of carbonaceous material, and 

 along the seams and cracks incrustations of gypsum and alkaline salts. 



