The composite sill in the Loch Scridain area of Mull, western Scotland, is typical of British Tertiary intrusions. Five members are recognized, ranging from tholeiite to felsite to tholeiite, with xenoliths in the lower and upper tholeiites. New chemical analyses of the felsite and a devitrified pitchstone in the felsite, and also of mullite and hercynite are given. The origin and emplacement are discussed.

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