In the narrow channels which the streams of water have sunk in it, may be seen perpendicular precipices of great elevation, but the valley between them is usually almost filled with large broken masses of the rock, which frequently exhibit a prismatic form. It falls readily into large masses, but seems strongly to resist that progress of disintegration which it must undergo before it can be removed by the water. The face of the perpendicular precipices are almost invariably marked by distinct and large seams running nearly parallel to each other, and at right angles with the horizon. Following the watercourses, which are sunk considerable distance below the surface, the line of separation from the sandstone on which the greenstone rests, at length becomes visible on account of the descent of the surface. 2. _Amygdaloid_, KIRWAN, JAMESON.--We apply this name to a porous or vesicular rock, of a very dark gray, greenish or black colour, usually found near the greenstone, but sometimes in connection with the sandstone. In its ultimate composition it resembles greenstone, but we have never seen in it such large fragments of felspar and scales of mica, as are observed in that rock. The amygdaloidal {292} cavities which every where penetrate this rock, are of various sizes, some of them appearing like bubbles which have been formed in a semifluid mass, and afterwards lengthened and variously distorted by the motions of the contiguous matter. Near the surface they contain a soft white, or yellowish white substance, very different from the rock itself, usually a soft chalk-like carbonate of lime. This gives the recent surface a mottled appearance. In surfaces which have been for some time exposed to the air, this soft substance has been removed, and the pores and vesicles are found empty. Amygdaloid does not appear to occupy any very great extent of the country near the Rocky Mountains. We have not met with it imbedded in, or surmounted by any other rock. Like the greenstone, it forms conic hills which sometimes occur in deep water-worn vallies, bounded on both sides by perpendicular walls of sandstone. It is likewise seen in the high plains, sometimes in the form of narrow and crooked ridges, apparently following what were anciently the beds of small brooks. Some very high and sharp conic hills were visible to the westward, but at a great distance. Two of this kind which stand near each other, and seem to be detached from the primitive mountains, are called the Spanish peaks, and at the end of July, snow was still to be seen on them.