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The oldest rocks of Scotland and of the British Isles are known, from their antiquity as Archean, and consist chiefly of gneiss (called Pundamental, as lying at the foundation of the geological structure of the country, and Leuisian and Hebridean, because it is well developed in the island of Harris and the Outer Hebrides), which varies from a coarsely crystalline granitoid mass to fine schist. The coarse varieties are most abundant intermingled with bands of hornblende-rock, hornblende-schist, pegmatite, eucrite, mica-schist, sericite-schist and other schistose accompaniments. In a few places limestone has been observed. No trace of any organism has ever been detected in any of these rocks.Over wide areas, particularly on the mainland, the bands of gneiss have a general north-west trend and undulate in frequent plications with variable inclination to north-east and south-west. The largest tract of Archean rock is that which forms almost the whole of the Outer Hebrides, from Barra Head to the Butt of Lewis. Other areas more or less widely separated from each other run down the western parts of the shires of Sutherland and Ross and Cromarty, and are probably continued at least as far as the island of Rum. |
