PERLITE or PEARLSTONE, a glassy volcanic rock which, when hammered, breaks into small rounded masses often of a pearly lustre, the reason being the many small cracks travers ing its glassy substance. These cracks mostly take a circular course, and often occur in groups, one within another; they bound the little spheres into which the rock falls when it is struck, and the concentric fissures reflecting light from enclosed films of air, are the cause of the pearly lustre. Longer straight cracks run across the sections separating areas in which the circular fissures preponderate. By decomposition the fissures may be occupied by secondary minerals ; the glass itself often undergoes change along the cracks by becoming finely crystalline or devitrified, dull in appearance and slightly opaque in section. In polarized light the perlitic glass is usually isotropic, but sometimes the interior of some of the spheres has a slight double refraction, apparently due to strain. Many rocks which are cryptocrystalline or felsitic, and not glassy, have perfect perlitic structure, and it seems probable that these were originally vitreous obsidians or pitchstones and have in time been devitrified to a finely crystalline state. Oc casionally in olivine and quartz rounded cracks not unlike perlitic structure may be observed.
Many perlitic rocks contain well-developed crystals of quartz, felspar, augite or magnetite, etc., and in the fine glassy base minute crystallites often abound. Some have the resinous lustre and the high percentages of combined water which distinguish the pitch stones; others are bright and fresh obsidians, and nearly all the older examples are dull, cryptocrystalline felsites. According to their chemical compositions they range from very acid rhyolites to trachytes and andesites, and the dark basaltic glasses or tachy lytes are sometimes highly perlitic. It is probable that most per
lites are of intrusive origin, as indicated by the general absence of steam cavities, but some perlitic Hungarian rhyolites are be lieved to be lavas. Rocks of this kind are found in Meissen, Saxony, as dikes of greenish and brownish pitchstone. Other ex amples are furnished by the Tertiary igneous rocks of Hungary (Tokay, etc.), the Euganean Hills (Italy) and Ponza Island (Mediterranean).
In mineralogical collections rounded nodules of brown glass varying from the size of a pea to that of an orange may often be seen labelled Marekanite. They are found at Okhotsk, Siberia, in association with a large mass of perlitic obsidian, and are the more coherent portions of a perlite. They are subject to considerable internal strain, and when struck with a hammer or sliced with a lapidary's saw they often burst into fragments—as do "Prince Rupert's drops." In their natural condition the marekanite spheres are doubly refracting, but when they have been heated and very slowly cooled they lose this property and no longer ex hibit any tendency to sudden disintegration.
In Great Britain Tertiary vitreous rocks are not common, but the pitchstone which forms the Scuir of Eigg is a dark andesitic porphyry with perlitic structure in its glassy matrix. A better example, however, is provided by a perlitic dacitic pitchstone porphyry that occurs near the Tay Bridge; the tachylytic basalt dikes of Mull are occasionally highly perlitic. (J. S. F.)