126

extinction on (001) of about 21-23 degrees and belongs therefore to bytownite. A nearly black very heavy rock, containing the same minerals as the preceding except olivine, seems to be a basic concretion of the olivine diabase. Pyroxene in sharp crystals tabular according to (100) predominates in the rock. It contains some plagioclase, mica, apatite and titanite.

ROCKS FROM THE DAVIS MOUNTAINS.

There are a number of elæolite and nepheline bearing rocks from the Davis mountains showing the labels: Fort Davis, Paisano Pass, Muerto camp, Sawtooth mountains, Mount Ord range, Railroad pasture, and Mosquez canyon. In part they are typical abyssic rocks belonging under the head of elæolite syenites and syenites, in part effusive rocks in various development, and lastly rocks occurring in dikes and distinguished from the two mentioned groups by their composition and structure. But they have been as yet but superficially examined. I hope, however, to have a chance to investigate them geologically and petrographically in a more minute manner.

Of the abyssic rocks, only the elæolite syenite from Paisano Pass has as yet been more thoroughly examined. It is a fine-grained light grayish white rock of uniform grain, showing a highly marked trachitic structure; the feldspars are tabular parallel to (010), and at the same time these planes in the rock are all lying parallel according to a fluidal arrangement; on the fracture across this plane the feldspars form small laths. Another constituent catching the eyes macroscopically is the elæolite ; although always changed to a dull white or reddish substance, it is nevertheless recognized by its six-sided and short rectangular sections, generally marked by a rim of dark minerals. These latter can not be determined without the microscope on account of their irregular shape and the fine grain of the rock. Isolated there are seen brightly glittering yellow grains, the nature of which was determined under the microscope as olivine.

Under the microscope the number of the constituents grows considerably. Besides the above mentioned (feldspar, elæolite, and olivine), there are now seen sodalite, ægyrite, a green augite, common hornblende, arfvedsonite, ainigmatite, laavenite, and, very seldom, an isotrophic mineral crystalizing in octohedrons and probably identical with the pyrrhite known from the Azores islands. In respect to the mineralogical composition of the rock, the characteristics are: 1. The entire absence of minerals of the mica group. 2. The rich development of minerals of the pyroxenic and amphibolic group. 3. The constant bearing of olivine. 4. The occurrence of some rare accessory minerals such as ainigmatite, laavenite and pyrrhite.