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The mountains composed entirely of sedimentary strata are remarkable for their great general uniformity of character. They occur mostly in continuous ranges, each one of which exhibits usually but a single slope, which is either towards the east or the west, being abruptly terminated in the opposite directions by bold vertical or nearly vertical escarpments, some of which, as for instance those forming the western faces of the Guadalupe and Sacramento Mountains, are nearly three thousand feet in height.

Inasmuch as the ranges composed altogether of sedimentary rocks are fully discussed in the Journal and report upon the Jornado del Muerto, besides being several times referred to in the chapters upon the Carboniferous System, I shall for the present confine myself entirely to such as are composed wholly or partially of eruptive rocks.

WITCHITA MOUNTAINS.

With the. exception of those occurring near the eastern extremity of the range, these mountains present a nearly uniform appearance and structure. Composed mostly of fine granite, they rise abruptly from a smooth and nearly level plain, and attain at their highest points an elevation of about nine hundred feet. Many of them are completely isolated, and of an irregular conical shape, while others are grouped together in clusters of eight or ten, and are more or less rounded. At a distance they appear to be smooth, but upon a nearer approach their surfaces are found to be quite rough, and present very much the appearance of loose rocks thrown confusedly together. This range is about seventy miles in length, and varies in width from five to fifteen miles. Its general bearing, as determined by Capt. R. B. Marcy, is about south 60 W.

At the base of these mountains the Marly Clay Formation (Lower Cretaceous) is exhibited in nearly horizontal layers, and presents generally but little evidence of disturbance, while the clays and sandstones have been found in several localities to contain inbedded fragments of rocks in all respects similar to those of the mountains. The period of eruption of these mountains must, therefore, date anterior to that of the deposition of the Lower Cretaceous Strata of the Plains.

LIMPEA MOUNTAINS.

The Limpea range has been only partially examined by myself. As already remarked, it is composed mostly of dark colored porphyry and basalt, and is evidently much more recent in origin than the range last considered, its protrusions occurring after the deposition of the limestone of the Upper Cretaceous group, which is found strongly upheaved against its northern base. These mountains exhibit at their highest points an elevation of about one thousand feet above the level of the surrounding country, and as