The cooling and hardening of hot, molten magma also takes place below
the earth’s surface. Here, the magma cools slowly to form rocks made up
of mineral grains that are large enough to be readily visible. These
rocks are known as _intrusive_ igneous rocks. We know that they are
present below the surface in Texas because of wells drilled in many
areas of the State. In Pecos County, a well reached granite, an
intrusive igneous rock, at a depth of 16,510 feet. Other wells in Texas
have reached the granite basement rocks at much shallower depths. But
not all intrusive igneous rocks in Texas are found underground. In the
Trans-Pecos country of west Texas, in the Balcones fault zone, and in
the Llano uplift of central Texas, some are now seen at the surface.
They, like all intrusive rocks, were formed below the ground, but
earth’s processes of uplift and erosion have gradually uncovered them.
[Illustration: Intrusive igneous rocks form from molten rock material
(magma) that cools and hardens beneath the earth’s surface.]