rhyolite

from WordNet (r) 3.0 (2006)

rhyolite
    n 1: very acid volcanic rock

from The Collaborative International Dictionary of English v.0.48

Rhyolite \Rhy"o*lite\, n. [Gr. "rei^n to flow + -lite.] (Min.)
   A quartzose trachyte, an igneous rock often showing a fluidal
   structure. -- {Rhy`o*lit"ic},, a.
   [1913 Webster]