South of the river the yellow strata are more distinctly developed. Butte Creek Valley, fifteen to eighteen miles to the south, is margined by bluffs of from twenty to one hundred and fifty feet in height on its southern side, while the northern rises gradually into the prairie. These bluffs are of yellow chalk, except from ten to forty feet of blue rock at the base, although many of the canyons are excavated in the yellow rock exclusively. The bluffs of the upper portion of Butte Creek, Fox, and Fossil Spring (five miles south) canyons, are of yellow chalk, and reports of several persons stated that those of Beaver Creek, eight miles south of Fossil Spring, are exclusively of this material. Those near the mouth of Beaver Creek, on the Smoky, are of considerable height, and appear at a distance to be of the same yellow chalk. I found these two strata to be about equally fossilliferous, and I am unable to establish any palæontological difference between them. They pass into each other by gradations in some places, and occasionally present slight laminar alternations at their line of junction. I have specimens of _Cimolichthys semianceps_ (Cope), from both the blue and yellow beds, and vertebrae of the _Liodon glandiferus_ (Cope) were found in both. The large fossil of _Liodon dyspelor_ (Cope) was found at the junction of the bed, and the caudal portion was excavated from the blue stratum exclusively. Portions of it were brought East in blocks of this material, and these have become yellow and yellowish on many of the exposed surfaces. The matrix adherent to all the bones has become yellow. A second incomplete specimen, undistinguishable from this species, was taken from the yellow bed. As to mineral contents, the yellow stratum is remarkably uniform in its character. The blue shale, on the contrary, frequently contains numerous concretions, and great abundance of thin layers of gypsum and crystals of the same. Near Sheridan concretions and septaria are abundant. In some places the latter are of great size and, being embedded in the stratum, have suffered denudation of their contents, and, the septa standing out, form a huge honey-comb. This region and the neighborhood of Eagle Tail, Colorado, are noted for the beauty of their gypsum-crystals, the first abundantly found in the cretaceous formation. These are hexagonal-radiate, each division being a pinnate or feather-shaped lamina of twin rows of crystals. The clearness of the mineral, and the regular leaf and feather forms of the crystals give them much beauty. The bones of vertebrate fossils preserved in this bed are often much injured by the gypsum formation which covers their surface and often penetrates them in every direction.