1. Greenstone, Jameson.—It appears in the limited district we examined under almost every variety of form and character noticed by mineralogists. Sometimes it is nearly or quite free from any intermixture of hornblende, is of a fine dark green colour, and closely resembles some varieties of serpentine. Sometimes its colour is a dull gray, graduating into brown and black of various shades and intensities. It forms numerous conic hills of considerable elevation, scattered without order, or grouped in various directions. These hills are usually of a regular and beautiful form. The great plain on which they are based is elevated and destitute of timber or water, but ornamented with a carpet of thick and verdant grasses. The hills, though steep and high, are sometimes smooth and green to the summit, the surface on all sides being unbroken by trees or rocks, and covered with thick turf. The whole forms a scene of singular {291} beauty. During our journey across the district, based upon the rocks now under consideration, we had constantly occasion to admire the freshness and abundance of the grasses and other herbaceous plants. The plains of the Platte and Arkansa we had seen brown and desolate, as if recently ravaged by fire; but here we passed elevated tracts, where, for many miles, we could find no water for our own necessities, yet the vegetation possessed the freshness of spring in the most fertile regions. But the conic hills just mentioned, are not invariably the form under which the greenstone appears. It sometimes rises in low irregular ridges, extending a considerable distance, and sloping on both sides into the level of the plain.
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.
When either of the two rocks last mentioned occur, it is not uncommon to find detached masses of a stone somewhat resembling the pumice-stone of commerce. It is usually of a faint red, or yellowish white colour, but sometimes it is brown, or nearly black. It feels less harsh than the pumice-stone which is used in the arts, and seems to consist in a great degree of clay. It appears to be entirely similar to the substance brought down the Missouri by the annual floods, and by many considered as a {293} product of pseudo-volcanic fires, said to exist on that river.
With regard to the soils resting upon the rocks of this trap formation, it may be worthy of remark, that gravel and water-worn pebbles rarely occur, except in situations where it is easy to see they may have been derived from the substratum of sandstone. We are not disposed to enter into any discussion concerning the origin of the trap rocks. The volcanists, and those who believe the trap formations to have been thrown up in a state of fusion from beneath the crust of the earth, will have an easy method of accounting for a fact mentioned in our journal, namely, that pieces of charred wood were found enclosed in the sandstone underlying the formation in question. Though we sought in vain for some evidence that the rocks of this formation traversed the strata of sandstone in the manner of the whin dikes of England, we are conscious our examinations were far too limited to justify us in asserting that this is not the case; nor can we adduce a single fact from which it could be inferred that these basaltiform rocks have been deposited, like the accompanying strata of sandstone, from suspension in water. The country occupied by this formation, exhibits scenery of a very peculiar and interesting character. It is remarked by Humboldt,[82] that "in the Canary islands, in the mountains of Auvergne, in the Mittelgebirge, in Bohemia, in Mexico, and on the banks of the Ganges," and we may add, in the United States, the formation of trap is indicated by a symmetrical disposition of the mountains by truncated cones, sometimes insulated, sometimes grouped, and by elevated plains, both extremities of which are crowned by a conical rising. In some of the unpublished drawings by Mr. Seymour, these peculiar {294} features of the scenery of the fletz trap formation, have been preserved.
RECAPITULATION
The secondary formations along the eastern base of the Rocky Mountains, are:
1st. Red Sandstone—Rests immediately upon the granite, is rather indistinctly stratified; strata sometimes inclined and sometimes horizontal; abounds in gypsum, salt, and iron, but exhibits no indications of coal.