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 ]

a. Relating to, or resembling, gneiss; consisting of gneiss. [ 1913 Webster ]

a. [ Gneiss + -oid. ] Resembling gneiss; having some of the characteristics of gneiss; -- applied to rocks of an intermediate character between granite and gneiss, or mica slate and gneiss. [ 1913 Webster ]

a. Having the structure of gneiss. [ 1913 Webster ]