The term country rock refers to a body of rock that receives or hosts an intrusion of a viscous geologic material. Intrusions into country rock are most commonly magmatic, but may also consist of unconsolidated sediments or salt horizons. Country rock may consist any other kind of rock that was present before the intrusion: sedimentary, igneous, or metamorphic.

In most cases, country rock is intruded by an igneous body of rock that formed when magma was forced upward through fractures or melted its way up through the overlying rock. The magma then cooled into solid rock forming a mass distinct from the enveloping country rock. Occasionally, a fragment of country rock will break off and become incorporated into the intrusion, and is called a xenolith.

The country rock is usually altered by the heat of the intrusion. The change that takes place in country rock as...