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The geological features of Trans-Pecos Texas are very interesting from a strictly scientific standpoint, and equally interesting and important on account of the rich and various mineral deposits, the existence of which can not be any longer disputed, though they are only sparingly developed in two places—the Shafter mine in the Chanatte mountains, and the Hazel mine at the foot of the Sierra Diablo in El Paso county.

The study of the geological features of Trans-Pecos Texas, from the strictly scientific point of view, needs, more than the study of other mountainous regions, the assistance of the microscope and of the chemical laboratory, because in most cases the connections and continuity of of the mountain ranges and groups are covered and hidden by extensive flats and basins filled 1500 and more feet with the debris of older and newer formations.

Dr. A. Osann joined the Geological Survey in the capacity of petrographer but last December, and he had only time to determine and classify a limited number of specimens microscopically. The required chemical analysis of many specimens was delayed by analyses of economical objects, and interrupted and stopped by the death of Dr. Melville, the chemist of the Survey, and so the progress of the strictly scientific study delayed.

We have to deal in Trans-Pecos Texas mostly with older and newer eruptive and numerous metamorphic rocks; the sedimentary strata reach, as far as determined till now, from the Silurian to the Cretaceous period; and Tertiary deposits, though not definitely determined, are not improbable.

In the Black Gulch of the Sierra Diablo the locally accessible substratum of the nearly horizontal carboniferous limestones is a grayish brown igneous rock, which Osann classifies as olivine diabase. About thirtyfive miles south of the Black Gulch, on the north side, and close to the Texas Pacific railroad in the Carrizo Pass, we meet again diabase unconformably overlaid by a brownish red grit without fossil remains, which stratigraphically may be termed Devonian, and thereon rests the Carboniferous limestone, with Euomphalus pentagonalis, Euomphalus nodosus,