TESCHENITE

coarse- to fine-grained, rather dark-coloured, intrusive igneous rock that occurs in sills (tabular bodies inserted while molten between other rocks), dikes (tabular bodies injected in fissures), and irregular masses and is always altered to some extent. It consists primarily of plagioclase feldspar, analcime, and titaniferous augite, with barkevikite, nepheline, and olivine usually in lesser amounts. The plagioclase crystals often are encased in the augite to give teschenite an ophitic texture. The rock is named for its occurrence near Teschen (now Cieszyn, Pol.); in central Scotland it is abundant in thick sills. Teschenite grades into picrite when the olivine content increases.