common and widely distributed type of rock
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]

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.

Lewisian gneissEdit

The Lewisian complex or Lewisian Gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane. These rocks are of Archaean and Palaeoproterozoic age, ranging from 3.0–1.7 Ga.[2]


  1. Blatt, Harvey, Robert J. Tracy & Ernest G Ehlers 1996. Petrology : igneous, sedimentary, and metamorphic. Freeman, New York.
  2. Ga = thousands of millions of years ago; so 1.7 Ga = 1,700,000,000 years ago