[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.
[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].