THE PORPHYRITIC ROCKS.
When the conditions are such that after a part of the magma has formed distinct crystals floating in the remaining liquid lava, there is a change which causes the rest to solidify as a glass or as a mass of small crystals, the structure is known as porphyritic, and the rocks possessing it are called porphyries. This differentiation into distinct crystals set in a ground-mass of minute crystals or of glass often gives a mottled or variegated aspect to the rock, especially if the matrix of glass or minute crystals differs in color from the distinct crystals. This structure is much oftener developed in acidic rocks than in basic ones, because the latter crystallize more readily. The most common porphyritic crystals are feldspar and quartz, though they are by no means the only ones. The matrix is also usually felsitic or quartzose, but not necessarily so. The character is a structural one, and is not dependent upon any special chemical or mineralogical constitution. The distinct crystals are known as phenocrysts, and the varieties of porphyries are named from the characteristic phenocryst, e.g., quartzophyre (quartz-porphyry) if the conspicuous crystals are quartz, orthophyre if orthoclase, augitophyre if augite, etc. A convenient classification has recently been proposed[200] into (1) leucophyre (white porphyries), which have a light-colored ground-mass set with phenocrysts of any kind, and (2) melaphyres (black porphyries), which have a dark-colored ground-mass, with phenocrysts of any kind. While it is to be hoped this usage will prevail, it is to be noted that these terms, especially the latter, have been used in a different sense. (See reference list of rocks, [p. 445].)
In many cases the ground-mass itself becomes minutely crystalline and the porphyritic aspect is due simply to large distinct crystals set in a mass of minute obscure ones. The rock is then really holocrystalline, but the term porphyry is applied to it. In other rocks the crystals of the ground-mass become more and more distinct, the porphyritic aspect gradually disappears, and there is a graduation into the next class.