EPIDOSITE  , in

petrology, a typical member of a family of metamorphic rocks composed mainly of epidote and quartz . In colour they are pale yellow or greenish yellow, and they are hard and somewhat brittle . They may occur in more than one way and are derived from several kinds of rock . Some have been epidotic grits and sandstones; others are limestones which have undergone contact-alteration; probably the majority, however, are allied to epidiorite and amphibolite, and are local modifications of rocks which were primarily basic intrusions or lavas . The sedimentary epidosites occur with mica-schists, sheared grits and granulitic gneisses; they often show, on minute examination, the remains of clastic structures . The epidosites derived from limestones may contain a great variety of minerals such as calcite, augite, garnet, scapolite, &c., but their source may usually be inferred from their close association with talc-silicate rocks in the field . The third group of epidosites may form bands, veins, or irregular streaks and nodules in masses of epidiorite and hornblende-schist . In microscopic section they are often merely a granular mosaic of quartz and epidote with some iron oxides and chlorite, but in other cases they retain much of the structure of the originar rock though there has been a complete replacement of the former minerals by new ones . Epidosites when streaked and variegated have been cut and polished as ornamental stones . They are translucent and hard, and hence serve for brooch stones, and the simpler kinds of jewelry . These rocks occasionally carry gold in visible yellow specks . (J .

S .