Abstract

Dr N. Holgate writes: Speight & Mitchell present what they regard as evidence to disprove the Tertiary dextral shift on the Great Glen Fault first recognized by Holgate (1969). However, they agree with him in recognizing a 7–8 km dextral shift on the main track of the fault between Morvern and Lismore, but contest its Tertiary dating.

They find no evidence of lateral displacement of Tertiary dykes at the Great Glen Fault and question the existence of its major splay passing SE of Lismore.

The writer agrees with the authors that the Tertiary Skye dyke-swarm intersects a Permo-Carboniferous swarm in the Morvern area. He also accepts their field criteria (op. cit., p. 5) for the recognition of Tertiary dykes within the area. But their statement (op. cit., p. 9) that Tertiary dykes are sparse on Lismore—11 dykes (which they arbitrarily assign to the Mull swarm) in the SW end of the island—is scarcely convincing, since their traverses, on Lismore and on the Appin coast (op. cit., p. 8; Fig. 3) exclude the several localities cited by Holgate (op. cit., p. 113) at which Tertiary dykes are displaced dextrally by NE–SW fractures marginal to the Firth of Lorne fault zone which separates Lismore from Appin. Towards the NE end of Lismore, such fractures become increasingly numerous as the Firth of Lorne shore is approached.

The Firth of Lorne fault diverges through some 10° in an anticlockwise sense from the Great Glen Fault at a point to the

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