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Webster 1913 Edition

Gneiss

Gneiss

(nīs)
,
Noun.
[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.

Webster 1828 Edition

Gneiss

GNE'ISS

,
Noun.
ne'is. In mineralogy, a species of aggregated rock, composed of quartz, feldspar and mica, of a structure more or less distinctly slaty. The layers, whether straight or curved, are frequently thick, but often vary considerably in the same specimen. It passes on one side into granite, from which it differs in its slaty structure, and on the other into mica slate. It is rich in metalic ores.

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