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The Slark comes from Joseph Geer's wife Mary Slark, grandparents of your grandfather Frank Geer.
The family came from Sussex rather than Exeter. I have traced back to John Geere of Wivelsfield near Haywards Heath in Sussex who would have been born about 1520.There may be a link with the Geere family of Exeter but if so it would be further back than 1520.