The summits of the Rocky mountains are formed entirely of primitive rocks, chiefly of granite itself. A red and saline sandstone rests on this granite, through the whole chain, as far as it has been explored. But few traces of that animal and vegetable life are found, which in other countries has reared mountains of limestone, clay-slate, and those other aggregates which are so often composed of the exuviæ of living beings. The western boundary of this sandstone formation corresponds to the side of the easternmost granite ranges. From the Platte toward the south, the sandstone increases in width, and on the Canadian it extends more than half the distance from the sources of that river to its confluence with the Arkansas. It consists of two members; red sandstone, and argillaceous or gray sandstone. This formation was at one time probably horizontal and uniform; it is now found in a state of entire disruption and disorder. This tract abounds in scenery of an interesting and majestic character. The angle of inclination of the strata varies from forty-five to ninety degrees. Though not very recent, the sandstone along the base of the mountains contains the relics of marine animals and plants, and embraces extensive beds of pudding stone. South of the Arkansas are rocks of basaltic origin, overlaying the red sandstone. By the vastness and broken character of their masses, and their dark color, they present a striking contrast to the light, smooth and fissile sandstone on which they rest. Sometimes they are compact and apparently homogeneous in their composition, and in many particulars of structure, form and hardness, more analogous to the primitive rock than to those recent secondary aggregates with which they are associated. In other instances, dark and irregular masses of porous and amygdaloidal substances are seen scattered about the plain, or gathered in conical heaps, but having no immediate connection with the strata on which they rest. Most of the rocks of this class were observed in the neighborhood of the sources of the Canadian; and may be distinguished into two kinds, referable to the two divisions called greenstone and amygdaloid.