1875.J ttUO [Stevenson. 



ceptibly into the Lignitic Group above. On Powder River one of the 

 lower sandstones of the latter group has layers hardened by the presence 

 of calcareous matter, so that the rock weathers into architectural forms, 

 the pillars being protected by a cap of the harder rock. 



On Gardiner's River the intimate relations of the two groups are well 

 shown. At one locality, where 1200 feet of strata, belonging to them are 

 exposed, it seems impossible to draw any line of division, "this great 

 group of beds, simply alternate beds of sandstone and arenaceous clays, 

 passing down into the dark sombre clays of the Cretaceous." At Cinna- 

 bar Mountain, above the mouth of Gardiner's River, "the dark, laminated 

 clays of the Cretaceous, passing up into the Upper Cretaceous, are well 

 shown with perfect continuity, then passing up into a great thickness of 

 the sombre brown sandstones of the Coal Group. There is a great uni- 

 formity between the Upper Cretaceous and Tertiary series. "We can de- 

 tect some variations in color and texture, but they are of minor import- 

 ance and could not easily be described in words."* 



On Box Elder Creek, not far from Fort Fetterman, the lignite series con- 

 sists of rusty sands and sandstones and arenaceous clays, with some seams 

 of lignite. On Deer Creek, twenty-seven miles from the Fort, the black 

 clays of No. 4 are capped by a thin bed of ferruginous arenaceous clays, 

 above which are two beds of sandstone. The lower one of these is concre- 

 tionary throughout; being filled with sandstone concretions imbedded in 

 an indurated clay, which also shows a tendency to concretionary struc- 

 ture. In the harder portions, a few specimens of Baculites, Inoceramus,. 

 etc., were found. The upper bed has a similar rusty-yellow color, but 

 yields no fossils. Both rocks, but especially the lower one, tend to weather 

 into architectural forms. Near old Fort Casper a yellow ferruginous 

 sandstone, containing Inoceramus and huge concretions, is seen res ting- 

 on black shaly clays which Dr. Hayden assigns to the horizon of Creta- 

 ceous, No. 2. 



On the North Platte River, from Sage Creek to Medicine Bow, and 

 thence to Bridger's Pass, the sandstones and the associated clays lying at 

 the base of the Lignite Group, are almost continuous. They rest direct- 

 ly upon Cretaceous clays. The sandstones are irregularly concretionary 

 and occasionaly yield an Inoceramus or Baculites. Some rusty calcare- 

 ous beds contain Ostrea. Along the Platte, four beds of the sandstone can 

 be distinguished. The first, second, and third, beginning at the base, are 

 in all fifty to eighty feet thick, drab-brown, and quite massive. The 

 fourth is yellowish-gray, full of large rusty-brown concretionary masses, 

 which ai'e laminated, and in reality are arenaceous limestones. Between 

 the beds are thin layers of sandstone and sandy limestones. At Cooper's 

 Creek the rusty arenaceous beds of No. 5 pass up gradually into the coal- 

 bearing layers without any perceptible bi*eak, and without any marked 

 change in the sediment. The latter series is from 1500 to 2000 feet thick 

 and consists of rusty-yellow sandstones, alternating with greenish-gray 

 * Hayaen. Report for 1871, p. 62. 

 A. P. S. — YOL. XIV. 3h 



