Basalt is a heavy igneous rock that is black, dark gray, or dark brown.
This rock is made up chiefly of a feldspar mineral, such as
_labradorite_, and a pyroxene mineral, such as _augite_. Other minerals
may be present.
The mineral grains of some basalts are so small that you cannot
distinguish them even with a magnifying glass. Other basalts, however,
are _porphyritic_, which means that they contain larger, easily seen
crystals and grains of feldspar and pyroxene scattered either through a
mass of the small mineral grains or through glassy material.
Some basalts contain many small holes. These holes, called _vesicles_,
were formed when bubbles of gas were trapped in the hardening magma.
Later, solutions moving through the rocks may have deposited another
mineral—such as calcite or chalcedony—in some of the vesicles.
Basalt forms from molten rock material that hardens either on or beneath
the surface—it can be extrusive or intrusive. Much of the basalt now
found in the Trans-Pecos country of west Texas formed from lava that
flowed out onto the surface during the Tertiary Period. A few of the
places where basalt occurs in west Texas are the Chinati Mountains of
Presidio County, the Chisos Mountains of Brewster County, the Davis
Mountains of Jeff Davis County, and the Van Horn Mountains of Culberson
and Hudspeth counties.
[Illustration: Basalt from Brewster County, Texas.]
Several varieties of basalt occur in the Balcones fault region of
Bandera, Comal, Hays, Kinney, Medina, Travis, and Uvalde counties. These
basalts formed from molten magma that forced its way into rocks just
below the earth’s surface.
Some basalt, which is known commercially as _trap rock_, is produced in
Uvalde County. It is crushed and used for railroad ballast, road
building material, and as concrete aggregate.
Bentonite. _See_ Clay.
Biotite. _See_ Mica.
Braunite. _See_ Manganese Minerals.