The Collaborative International Dictionary
Tachylyte
Tachylyte \Tach"y*lyte\, n. [Gr. tachy`s quick + ? to dissolve.] (Min.) A vitreous form of basalt; -- so called because decomposable by acids and readily fusible.
Wiktionary
tachylyte
n. (context geology English) A black, vitreous basalt of volcanic origin