Scientific editing by Richard England.

D. M. D. James writes: The paper is much to be welcomed as bringing new and stimulating ideas to the ongoing debate concerning the origin of polygonal faults (Cartwright & Lonergan 1996; Cartwright & Dewhurst 1998). However, if the proposed mechanism of blind faulting above sites of incipient diapirism is correct, then demonstration is required of significant thickness variation of the causative mobile layer and of a density inversion across its upper boundary. The following remarks aim largely to seek geophysical reassurance on these aspects, and as such are also relevant to many of...

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