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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.

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 third group of epidosites may form bands, veins, or irregular streaks and nodules in masses of epidiorite and hornblende-schist.

The metamorphic rocks are associated with bands of epidiorite which have shared in the folding and metamorphism of the region.

The older acid rocks are represented by the sills of granite and augen-gneiss occurring west of Portsoy, south of Fordyce and near Keith, while the older basic rocks are illustrated by the belt of gabbro, epidiorite and hornblende-schist which stretches southwards from the coast at Portsoy, by Rothiemay to Huntly in Aberdeenshire.