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The Collaborative International Dictionary of English v.0.48 (gcide)

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]