The general appearance of the islands is not unlike that of one of the outer Hebrides.
They have been broadly classified into the Outer Hebrides and the Inner Hebrides, the Minch and Little Minch dividing the one group from the other.
The Outer Hebrides being almost entirely composed of gneiss the epithet suitably serves them, but, strictly speaking, only the more northerly of the Inner Hebrides may be distinguished as Trap Islands.
Dispersed over all parts of the western Highlands, they are most numerous in the north-west, especially in the Outer Hebrides and in the west of the shires of Ross and Cromarty and Sutherland, where the surface of the Archean gneiss is so thickly sprinkled with them that many tracts consist nearly as much of water as of land.
In the Outer Hebrides most of the ground is low, rocky and plentifully dotted over with lakes; but it rises into mountainous heights in Harris, some of the summits attaining elevations of 2600 ft.