Igneous layers are either plutonic or volcanic in character depending upon the cooling rate of the rock.
From the en.wikipedia.org
A rock that cools within the Earth is called intrusive or plutonic and will cool very slowly, producing a coarse-grained texture.
From the en.wikipedia.org
For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least via microscope, the mineralogy is used to classify the rock.