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Most of the eruptive rocks encountered by myself in Texas and New Mexico are described in detail in the Journal and accompanying report upon the Jornado del Muerto. I shall, therefore, in this place only notice those of the Limpea Mountains, and such others as were examined during my former trip to the sources of Red River. The general distribution of this class of rocks in the region under consideration will be more particularly noticed when I come to speak of mountain ranges.

Eruptive Rocks of the Limpea Mountains.-These, as far as examined by myself, consist mostly of claystone and feldspar porphyries and dark colored basalt. The porphyries are hard, compact, and of purple, red, brown, and gray colors. Their crystals are usually well defined, and consist for the most part of labradorite, quartz, and feldspar. Those composed of labradorite are by far the most abundant, and usually exhibit in great perfection the characteristic change of colors of that mineral. The basalt is only observed near the northern base of the mountains. In some places it is hard and very compact, while in others it is soft or more or less vesicular, sometimes passing into vesicular lava. All these rocks appear to be yielding rapidly to the weather, and are traversed in almost every direction by deep fissures or canyons, some of which are many miles in length. Externally they are of reddish brown and black colors, and not unfrequently their weathered surfaces present a highly glazed or polished appearance.

Eruptive Rocks of the Witchita Mountains -The rocks of the Witchita Mountains consist of hard, compact granite and porphyry, with occasional veins of greenstone, hornblende, and cellular and compact quartz. The granite is far more abundant than the porphyry, and composes the great mass of the formation. It is usually fine textured, of a deep red color, and contains a great predominance of feldspar and a deficiency of mica, the latter mineral being in many instances almost entirely absent. Not unfrequently the rock passes into nearly pure feldspar. Externally it is of deep brown, yellow, or red colors, and owing to the great number of joints and master joints by which it is traversed, presents somewhat of a cuboidal structure.

The porphyry is confined almost exclusively to the eastern extremity of the mountains, where it is exhibited in gently rounded hills and ridges from three to five hundred feet high. It is also of a deep red color, but is usually much softer than the granite. Its crystals are mostly small and composed of flesh-colored feldspar. The veins of greenstone, hornblende, and quartz are sometimes many feet in width, and traverse the mountains in almost every