PORPHYRITIC a.
Relating to, or resembling, porphyry, that is, characterized by the presence of distinct crystals, as of feldspar, quartz, or augite, in a relatively fine-grained base, often aphanitic or cryptocrystalline.
PORPHYRY n.
to designate a rock consisting of a fine-grained base (usually feldspathic) through which crystals, as of feldspar or quartz, are disseminated. There are red, purple, and green varieties, which are highly esteemed as marbles. Porphyry shell (Zoöl.), a handsome marine gastropod shell (Oliva porphyria), having a dark red…
POSITIVE a.
not acid; -- opposed to negative, and said of metals, bases, and basic radicals. Positive crystals (Opt.), a doubly refracting crystal in which the index of refraction for the extraordinary ray is greater than for the ordinary ray, and the former is refracted nearer to the axis than the latter, as quartz and ice; -- op…
PRIMARY a.
mary rocks (Geol.), a term early used for rocks supposed to have been first formed, being crystalline and containing no organic remains, as granite, gneiss, etc.; -- called also primitive rocks. The terms Secondary, Tertiary, and Quaternary rocks have also been used in like manner, but of these the last two only are no…
PRISMATIC; PRISMATICAL a.
Same as Orthorhombic. Prismatic borax (Chem.), borax crystallized in the form of oblique prisms, with ten molecules of water; -- distinguished from octahedral borax. -- Prismatic colors (Opt.), the seven colors into which light is resolved when passed through a prism; primary colors. See Primary colors, under Color.…
PROTOCATECHUIC a.
Pertaining to, derived from, or designating, an organic acid which is obtained as a white crystalline substance from catechin, asafetida, oil of cloves, etc., and by distillation itself yields pyrocatechin.