nTachylyte(Min) A vitreous form of basalt; -- so called because decomposable by acids and readily fusible.
Century Dictionary and Cyclopedia
ntachylyteA vitreous form of basalt; basalt-glass; a rock occurring frequently along the edges or selvages of dikes of basalt or other kinds of basic lava, but sometimes forming flows of considerable magnitude, as at Kilauea. Tachylyte does not have so conchoidal a fracture as obsidian; it is much more fusible, and contains more water than that variety of volcanic glass. The proportion of silica in tachylyte varies from 50 to 55 per cent.; that in obsidian runs from 60 to 80 per cent.