extrusive: finely crystalline or glassy igneous rock resulting from volcanic activity at or near the surface (cools very quickly!).

felsic: a light colored mineral (often quartz or feldspar)

intrusive: pertaining to material forced, while still in a fluid state, into cracks or between layers of rock (cools very slowly!).

mafic: a mineral containing predominantly ferromagnesian rock forming silicates.  In general, a dark mineral.

matrix: the continuous, fine-grained material in which large grains are imbedded.

phenocryst: a large, conspicuous crystal in a porphoritic rock.

porphoritic: an igenous rock in which large phenocrysts are enclosed in a very fine-grained matrix.