\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 or
syenitic gneiss}. Similar varieties of related rocks are also
called gneiss.