PERCIVAL ALLEN, JOHN SUTTON, JANET V. WATSON; Torridonian tourmaline-quartz pebbles and the Precambrian crust northwest of Britain. Journal of the Geological Society ; 130 (1): 85–91. doi: https://doi.org/10.1144/gsjgs.130.1.0085
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Exotic rocks found as pebbles in the Torridon Group span the range of Lewisian events (3000 to 1000 my. B.P.) but were formed at higher crustal levels than the present Lewisian. Their variations southward along the present outcrop seem to reflect changes in the basement as it is traced through Greenland towards Labrador. The new evidence is consistent with the views of previous workers. Tourmaline is used as a 'supracrustal index' and confirmed as a valuable dating mineral.