Tachylyte n. : A vitreous form of basalt; -- so called because decomposable by acids and readily fusible.

Example:


Taxonomy
tachylite, Basalts, olivine, rocks, palagonite, glass, masses, augite, crystals, occurrence, spherulites, brittle, free encyclopedia, tachylites, vesicular, spherulitic, tuffs, dolerite.

Defination from the web:
Tachylyte) Jump to: navigation, search. Tachylite (also spelled tachylyte) is a vitreous form of basaltic volcanic glass. This "glass" is formed naturally by the rapid cooling of molten basalt. ...