Colonsay and the western part of Islay, southern Hebridean islands of Scotland, consist of unfossiliferous, deformed, low-grade metasedimentary rocks, which rest with sheared unconformity upon gneissic basement. The area has a long history of investigation, but its stratigraphic affinities have been a persistent problem.

The speculative assignment of the basement rocks to the Lewisian has never been substantiated. The age of the cover sequence remains unclear. Its deformation differs from that of nearby Caledonian successions. Small igneous bodies have yielded a tentative intrusion age of ca. 600 Ma, which is abnormal for the region. Because the area is bounded by major strike-slip faults, it is suggested that the Colonsay-western Islay terrane is allochthonous, probably emplaced between 620 and ca. 540 Ma.

The terrane concept explains the long-standing difficulties of correlation. Recognition of suspect terranes, even of this small size, within orogens such as the orthotectonic Caledonides can help resolve some long-standing problems.

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