Gneiss is a metamorphic rock that has parallel layers or bands. Some
gneiss is made up of the same minerals (chiefly feldspar and quartz) as
granite, and it is then called _granite gneiss_. Several of the other
kinds of gneiss are known as _mica gneiss_, _conglomerate gneiss_,
_gabbro gneiss_, and _hornblende gneiss_. In order to be a gneiss, a
metamorphic rock has to have bands or layers. These bands may be either
straight or wavy and either wide or narrow. In most gneisses, you will
find a layer made up of long or flat mineral grains next to a layer made
up of the grains of an entirely different mineral. The bands may show
color differences, too. For example, a pink layer made up of feldspar
grains may be found next to a black layer made up of hornblende grains.
The mineral grains interlock as they do in igneous rocks, and they are
generally large enough to be seen without a magnifying glass.
[Illustration: Gneiss from Blanco County, Texas, showing light and dark
bands.]
Gneiss can form from an igneous rock, such as granite, or from a
sedimentary rock, such as sandstone. Heat, fluids, and pressures below
the earth’s surface change these rocks into gneiss.
Gneiss that formed during Precambrian time is now seen at the surface in
both west Texas and central Texas. In west Texas, it occurs principally
in the Van Horn area of Culberson and Hudspeth counties. In central
Texas, it is found in Blanco, Burnet, Gillespie, Llano, and Mason
counties of the Llano uplift area.
One of the Llano uplift rocks is called the Valley Spring Gneiss. It
generally has a light color (much of it is pinkish), and it is believed
to have once been a sandstone. Another gneiss of this area, the Big
Branch Gneiss, which has a medium to dark gray color, occurs in northern
Gillespie and Blanco counties and is an altered igneous rock. Some of
the Texas gneiss rocks are suitable for use as building stones.