EPIDIORITE  , in

petrology, a typical member of a family of rocks consisting essentially of hornblende and felspar, often with epidote, garnet, sphene, biotite, or quartz, and having usually a foliated structure . The term is to some extent synonymous with " amphibolite " and " hornblende-schist." These rocks are metamorphic, and though having a mineral constitution somewhat similar to that of diorite, they have been produced really from rocks of more basic character, such as diabase, dolerite and gabbro . They occur principally among the schists, slates and gneisses of such districts as the Scottish Highlands, the north-west of Ireland, Brittany, the Harz, the Alps, and the crystalline ranges of eastern N . America . Their hornblende in microscopic section is usually dark green, rarely brownish; their felspar may be clear and recrystallized, but more frequently is converted into a turbid aggregate of epidote, zoisite, quartz, sericite and albite . In the less complete stages of alteration, ophitic structure may persist, and the original augite of the rock may not have been entirely replaced by hornblende . Pink or brownish garnets are common and may be an inch or two in diameter . The iron oxides, originally ilmenite, are usually altered to sphene . Biotite, if present, is brown; epidote is yellow or colourless; rutile, apatite and quartz all occur with some frequency . The essential minerals, hornblende and felspar, rarely show crystalline outlines, and this is generally true also of the others . The rocks may be fine grained, so that their constituents are hardly visible to the unaided eye; or may show crystals of hornblende an inch in length . Their prevalent colour is dark green and they weather with brown surfaces .

In many parts of the

world epidiorites and the quartz veins which sometimes occur in them have proved to be auriferous . As they are tough, hard rocks, when fresh, they are well suited for use as road-mending stones . (J . S .