The Collaborative International Dictionary of English v.0.48:

Gneiss \Gneiss\ (n[imac]s), n. [G.] (Geol.)
   A crystalline rock, consisting, like granite, of quartz,
   feldspar, and mica, but having these materials, especially
   the mica, arranged in planes, so that it breaks rather easily
   into coarse slabs or flags. Hornblende sometimes takes the
   place of the mica, and it is then called hornblendic gneiss
   or syenitic gneiss. Similar varieties of related rocks are
   also called gneiss.
   [1913 Webster]