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This range, as already remarked, is composed partly of eruptive and partly of sedimentary rocks. The eruptive rocks consist mostly of hard, compact, red and gray porphyritic granite and red porphyry. Those towards the south are seen rising in sharp jagged peaks to the height of near three thousand feet, and with some local exceptions constitute all that portion of the range lying between El Paso and the San Augustine Pass. Immediately north of this pass the eruptive rocks are seen to descend rapidly, and at the distance of a few miles farther north are exhibited only in low detached hills, their place in the mountains becoming occupied by upheaved strata of the Carboniferous System, which last soon attain an elevation of about three thousand feet, and thence northward constitute the principal portion of the Organ range until we arrive at the northern extremity of the Jornado del Muerto, beyond which point my examinations have not extended.

Wherever the sedimentary strata prevail the mountains exhibit but a single slope, which is uniformly towards the west, being abruptly terminated in the opposite direction by lofty escarpments, many of which are nearly vertical. So far as ascertained, the most recent strata elevated by these mountains are those of the Coal Measures. The period of eruption and of upheaval must, therefore, have occurred subsequent to the deposition of these strata.

Besides the mountains above described, a volcanic range of much more recent origin has been traced for several hundred miles along the Rio Grande. The general direction of this range appears to be pretty nearly north and south. I have myself encountered it at several points between El Paso and the Fra Cristoval Mountain. By Mr. Marcon and others it is mentioned as occurring as far north as Albuquerque and Santa Fe, while Mr. Thompson, a member of our own party, informs me that he has traced it in a southerly direction to the Laguna Patos, nearly a hundred miles south of El Paso. The range, as far as examined by myself, is formed altogether of isolated conical hills, which vary in height from three to six hundred feet, and are composed entirely of dark greenish compact basalt and vesicular lava. From each of these cones streams of dark colored vesicular lava, varying in width from a fourth to one or two miles, and from thirty to seventy feet in thickness, are seen to have issued. Several of these cones and lava streams are described in detail in the Journal and report upon the Jornado del Muerto. The lava appears to have undergone but little alteration since the period of eruption, its upper surface being black, highly vesicular, and totally devoid of vegetation, and from the fact of its being usually found resting above the drift is evidently of more recent origin than the latter.