123

REPORT
ON THE
ROCKS OF TRANS-PECOS TEXAS.

By A. Osann.

The collections of the Geological Survey contain a series of rocks collected in Trans-Pecos Texas by Prof. von Streeruwitz on his travels for exploration of the ore deposits during the last three years. It has not been possible during my short stay in the Geological Survey to work up a detailed investigation of that material, but only to give a brief preliminary report; partly because the thin sections for microscopical examinations of all specimens could not be gotten ready; partly because no chemical tests or analyses have been made of the rocks or their constituents. Besides, there are wanting special notes in regard to the occurrence of these rocks in the field. Everybody familiar with the progress of Petrography during the last years, knows that besides the microscopical investigation on the one hand, the chemical in the laboratory, and the geological in the field, on the other, are absolutely necessary for the right understanding of rocks. If, notwithstanding, some unconnected and incomplete notes on that subject are given, it is done in accordance with the order of the director of the Geological Survey, with a view to giving an idea of the great variety, especially of igneous rocks in Western Texas, and for the reason that no petrographical description of them has yet been given.

The geological material collected in Western Texas can be divided into four groups:

• 1. Igneous Rocks.
• 2. Crystalline Schists.
• 3. Sedimentary Rocks.
• 4. Ore Specimens.

Only the first group will be described in this report. Of crystalline schists there are only a few specimens from "Mica Tanks" in the Van Horn Mountains. They consist of gneisses (muscovite gneiss and muscovite and biotite-bearing gneiss), mica schists and amphibole gneisses in transition to amphibolites. These rocks are not yet studied under the microscope.