Granite is an intrusive igneous rock that is made up chiefly of
crystalline grains or crystals of quartz and a feldspar mineral, such as
orthoclase or microcline. Several other minerals, including mica and
hornblende, may also be present.
All of the mineral grains in granite are about the same size, and you
can distinguish them without using a magnifying glass. A granite may be
coarse grained, medium grained, or fine grained. When you examine this
rock, you will see that its grains are not cemented but interlocked like
the pieces of a jigsaw puzzle. The color of granite, which is pink, red,
gray, or brownish, depends chiefly on the color of its feldspar grains.
Most granites formed from hot, molten magma that slowly cooled and
hardened far below the earth’s surface. Because of this slow cooling,
fairly large mineral grains were formed.
[Illustration: Polished section of pink granite from Gillespie County,
Texas.]
Granites are now seen at the surface in several areas of Texas. They
were gradually uncovered as the areas became higher and the overlying
rocks slowly weathered away. One of these areas is the Llano uplift of
central Texas where the granites occur in Blanco, Burnet, Gillespie,
Llano, and Mason counties. These granites formed during Precambrian time
and are believed to be about a billion years old. (Scientists are now
able to determine the age of some rocks accurately by very precisely
measuring the relative amounts of isotopes produced by decay of
radioactive minerals.)
[Illustration: Texas State Capitol building at Austin is made of Burnet
County granite obtained from Granite Mountain near Marble Falls, Texas.]
Granites also appear at the surface in the Trans-Pecos country of west
Texas. Some of these areas include the Franklin Mountains of El Paso
County, the Quitman Mountains of Hudspeth County, the Chisos Mountains
of Brewster County, and the Chinati Mountains of Presidio County.
Red, pink, and gray granites from quarries in the Llano uplift area are
widely used as building stones and monument stones. A large quarry at
Granite Mountain just west of Marble Falls in Burnet County has supplied
pink granite for buildings in many parts of the United States. The Texas
Capitol building and several other State buildings in Austin are made of
this granite.