Felsite

FELSITE, in petrology, a term which has long been generally used by geologists, especially in England, to designate fine grained igneous rocks of acid (or subacid) composition. As a rule their ingredients are not determinable by the unaided eye, but they are principally felspar and quartz as very minute par ticles. The rocks are pale-coloured (yellowish or reddish as a rule), hard, splintery, much jointed and occasionally nodular. Many contain porphyritic crystals of clear quartz in rounded blebs, more or less idiomorphic felspar, and occasionally biotite; others are entirely fine-grained and micro- or crypto-crystalline; occasionally they show a fluxional banding; they may also be spherulitic or vesicular. Those which carry porphyritic quartz are known as quartz-felsites; the term soda-felsites has been applied to similar fine-grained rocks rich in soda-felspar.

Although there are few objections to the employment of felsite

as a field designation for rocks having the above characters, it lacks definiteness, and has been discarded by many petrologists as unsuited for the exact description of rocks, especially when their microscopic characters are taken into consideration. Never theless the term is an extremely useful one in field geology, as a preliminary description of rocks; both intrusive and volcanic, whose precise nature can later be determined by the usual petro graphical methods in the laboratory.