One thin bed in sandstones belonging to the Stoer Group (Torridonian Supergroup) contains feldspar and quartz grains showing deep concave embayments which are interpreted as corrosion features, and are all embedded in a fine-grained illitic or sericitic matrix. Its bulk chemical composition is similar to that of Phanerozoic metabentonites but it differs from the overlying Stac Fada Member which is believed to represent either a volcanogenic deposit or an impact ejecta blanket. Field relationships, petrographic and chemical characteristics point to this bed having a volcanic origin. This thin bed represents a crystal tuff and is the earliest example of a volcanogenic deposit in the Torridonian Supergroup in Scotland.

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