Although a number of discoveries have been made in recent years, there are very few historical reports of volcanic ash deposits in the Torridonian rocks of Scotland. New examples extend the known occurrence of such deposits and analyses suggest a geochemical stratigraphy for a suite of rocks that are otherwise notoriously difficult to correlate. Tuffs from the Colonsay Group are described for the first time, subdivided according to their geochemical characteristics. The Kilchattan Formation of Colonsay may correlate with the Torridonian Diabaig Formation and, based on the geochemistry of both tuffs and contemporary phosphorites, the Upper Diabaig Formation may be correlated with the Lower Sleat Group. This is the first attempt to construct a stratigraphy of the Torridonian based on the geochemical characteristics of contemporary tuffs.

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