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.
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.
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.
Images for kids
Dark dikes (now foliated amphibolites) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes
Orthogneiss from the Czech Republic