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From Mica Tanks there is also a very fresh black vitrophyre, presenting under the microscope phenocrysts of a glassy feldspar, quartz, and a pale green pyroxene in a brownish colored glass.
An interesting fine-grained rock from the railroad near Van Horn shows macroscopically an aggregate of white grains and a black mineral in about equal amounts; under the microscope the former is seen to be quartz, the latter to be tourmaline. Quartz forms an angular mosaic of grains giving a honey-combed structure as it is characteristic of contact rocks (hornfels). Many of the grains present a very regular sixsided shape, but the examination between crossed nicols shows that these sections are of different orientation and not cut perpendicular to the c axis. The mineral contains few inclusions of liquid and shows no traces of a clastic origin or a later overgrowth; a cement between the grains is wanting entirely. Tourmaline occurs in prismatic crystals of about 0.5 mm. in length and 0.03-0.1 mm. in width; these columns are not scattered uniformly through the rock, but form radiated aggregates as it is the case in the well known luxullianite. The pleocroism of the tourmaline is the following:
- e. Light brown violet.
- o. Dark gray blue.
In the vicinity of these tourmaline aggregates there is found another mineral forming small prismatic crystals or grains of a very high index of refraction and double refraction. The mineral presents a yellow or light brown color, and has an appearance like rutile. In all likelihood this mineral is cassiterite, but it has not yet been possible to isolate it and make the test for SnO2.
The whole mineralogical combination and the structure of the rocks bring it under the head of tourmaline hornfels, a group of rocks made by means of mineralizing agents in the contact zone of acid, abyssic rocks.
ROCKS OF THE CHISOS MOUNTAINS.
A holocrystalline dacite rich in phenocrysts of feldspar, biotite, and hornblende. Phenocrysts of quartz are scarce and only seen under the microscope. The rock in its appearance is very similar to the dacites of the Esterell mountains in southern France. Under the microscope the ground mass is holocrystalline and is made up of an allotriomorphous aggregate of quartz and feldspar.
From the Bofecillos mountains there is one specimen of a highly scoriaceous black augite andesite showing the common properties of these rocks.