Magma is typically less dense than the rock it intrudes, widening and filling existing cracks, sometimes melting the already-existing country rock.
Country rock can denote the widespread lithology of a region in relation to the rock which is being discussed or observed.
Settings in geology when the term country rock is used include:
When country rock is intruded by dyke, perpendicular to the bedding plane, it is called discordant intrusion, while a parallel intrusion by a sill indicates a sub-parallel or concordant intrusion.
Most intrusions into country rock are via magma. Usually, country rock is intruded by an igneous body of rock which formed when magma forced upward through fractures, or melted through overlying rock. Magma then cooled into solid rock, different from the surrounding country rock. Sometimes, a fragment of country rock will break off and become incorporated into the intrusion, and is called a xenolith, from Greek, ξένος, xenos, "strange,", and λίθος, lithos, the ancient Greek word for "stone." 
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