The great alluvial formation, which occupies the country on both sides of the lower portion of the river Platte, is an almost unmixed siliceous sand, in no manner distinguishable from the _débris_ of the sandstones of transition mountains. Near the forks of the Platte, we first observed that the waters of that river bring down, among other matters, numerous small scales of mica. This also is a constituent of the sandstones of the lower secondary or transition formations. The fragments of unmixed and crystalline felspar, which now began to be of frequent occurrence, were considered as the first convincing evidence of the primitive character of the Rocky Mountains. These fragments of felspar, we believed, could have been derived from no other than primitive rocks. During all the day on the 23d we travelled along the south side of the Platte, our course inclining something more towards the south-west than heretofore. Intermixed in the narrow fringe of timber, which marks the course of the river, are very numerous trees, killed by the action of the beaver or by the effects of old age; their decorticated and bleached trunks and limbs strongly contrasting with the surrounding objects, many of them rendered doubly {156} interesting by affording a support to the nests of the bald eagle, elevated like a beacon in the horizon of the traveller. Large herds of bisons were seen in every direction; but as we had already killed a deer, and were supplied with meat enough for the day, none of the party were allowed to go in pursuit of them. Prickly pears became more and more abundant as we ascended the river; and here they occurred in such extensive patches as considerably to retard our progress, it being wholly impracticable to urge our horses across them. The cactus ferox is the most common, and, indeed, the only species which is of frequent occurrence. It has been stated by a traveller to the Upper Missouri,[108] that the antelope, which inhabits the extensive plains of that river and its tributaries, finds means to make this plant, notwithstanding its terrific armature of thorns, subservient to its necessities, "by cutting it up with his hoofs." We were able to discover no confirmation of this statement; it may, however, be applicable to some plains more arid and sterile than any we have passed, where the antelopes may be driven by necessity to the use of this hard expedient. [108] Nuttall's Genera of North American Plants, vol. i. p. 296.—JAMES. On the following day we saw immense herds of bisons, blackening the whole surface of the country through which we passed. At this time they were in their summer coat. From the shoulders backwards, all the hinder parts of the animal are covered with a growth of very short and fine hair, as smooth and soft to the touch as a piece of velvet. The tail is very short, and tufted at the end, and its services, as a fly-brush, are confined to a very limited surface.