Final answer:
The reaction of country rock with solutions from a cooling magma is known as hydrothermal alteration, which is a part of the metamorphic rock formation process.
Step-by-step explanation:
The reaction of country rock with solutions from a cooling magma is known as hydrothermal alteration. This process is part of the formation of metamorphic rocks, where pre-existing rocks undergo changes through heat, pressure or interaction with chemically reactive fluids without melting.
During hydrothermal alteration, hot, mineral-rich fluids interact with the surrounding solid rock (protolith), altering its mineralogical composition, texture, and chemical structure. This process can happen in the vicinity of magmatic bodies where fluid circulation is enhanced by the heat from magma.