412 GEOLOGY. 



is a change which causes the rest to soHdify 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 por- 

 phyritic 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 dis- 

 tinct 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 ^ 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. AVhile 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 apphed 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. 



THE PHANEROCRYSTALLINE ROCKS. 



The phanerites. — When time enough is given for the cooling process 

 the molten magma becomes completely crystalUne. The holocrystal- 

 Hne rocks hence include a large series, ranging from the most acid to the 

 most basic. In this class the differentiation of the rock material and 



^Quantitative Classification of Igneous Rocks, by Whitman Cross, Joseph P. 

 Iddings, Louis V. Pirsson, and Henry S. Washington. 1903. 



