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Gneiss

Outcrop of weathered Lewisian gneiss, 5 km NW of Loch Inver, Isle of Lewis
Gneiss rock
The stone circle at the centre of the Standing Stones of Callanish ("Callanish I"), Isle of Lewis. Reputed to be, at 3 billion years, the oldest rocks in the UK.

Gneiss is a type of metamorphic rock. The minerals in gneiss may come from rocks which were originally either igneous or sedimentary. They were heated and squeezed, and the minerals recrystallized.[1]

Orthogneiss is gneiss got from igneous rock (such as granite). Paragneiss is gneiss got from sedimentary rock (such as sandstone).

In gneisses, minerals tend to be foliated: layered and segregated into bands. Thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral.

  1. ↑ Blatt, Harvey, Robert J. Tracy & Ernest G Ehlers 1996. Petrology : igneous, sedimentary, and metamorphic. Freeman, New York.
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