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far as examined by myself present everywhere a dark and exceedingly rugged appearance. In some instances they are completely isolated, and are seen rising in the form of truncated cones; in others their summits are sharp and jagged, and from their peculiar shape are sometimes compared not very inaptly to a bishop's mitre. Most frequently, however, they are grouped together in short detached ranges, most of which appear to have a general north and south bearing, and are usually marked on all sides by abrupt and in many instances nearly vertical walls, which sometimes exhibit a slightly columnar appearance.

Under the name of Hueco Mountains are included all igneous protrusions encountered by the Expedition between the sounthern extremities of the Guadalupe and Sacramento Mountains as well as those described in the Journal as occurring farther north along the western base of the Sacramento Mountains.

This range consists altogether of isolated peaks, the highest of which, the Sierra Alto, is about fifteen hundred feet above the level of the surrounding plain. Towards the south they are generally much more elevated than at the north, and pursue a very irregular course, their general direction being there pretty nearly W. S. W. Near the Sierra Hueco, however, they are seen to bend abruptly towards the north, and are afterwards traced in low hills and ridges for a considerable distance along the western base of the Sacramento Mountains. As far as examined they are, with a single exception (viz., Sierra de los Alamos), composed entirely of fine compact granite, containing a much larger percentage of feldspar than usual, and a deficiency of mica. Around the base of several of them the limestone and sandstone of the Coal Measures are found metamorphosed and strongly upheaved. The protrusion of these mountains must, therefore, have occurred after the close of the Carboniferous Period.

The Mimbres Mountains form a nearly continuous range, whose general bearing is pretty nearly north and south. As far as examined they appear to attain their greatest elevation at the Picache de los Mimbres, a sharp conical peak situated near their southern extremity, from which point there is a gradual descent towards the north. At the " Picache," the only portion of the range visited by myself, the principal rock was found to be a hard and very compact porphyritic granite, which composed the main axis, with porphyry, quartz, vesicular amygdaloid, and chalcedonic rocks on either side. No sedimentary rocks were anywhere observed in the neighborhood. I am, therefore, unable to form an opinion with regard to the relative age of these mountains.