Dr W. E. A. Phillips writes: I would like to draw attention to a number of serious objections to the observations and conclusions presented by Crow et al. 1971 in their account of the metamorphic rocks of NW Co. Mayo.

One of the authors' main conclusions was that a major fault, 'the Corraun Fault' could be recognized, that it separated blocks of contrasted stratigraphy and structure, and that the fault could be regarded as the SW continuation of the Leannan Fault in Donegal (Pitcher et al. 1964). The direct evidence for the Curraun fault can only be seen in a gully SE of Corraun Hill, where a fault plane is exposed. Careful mapping would have shown that the displacement here can be no more than a few hundred metres at the most, for lithostratigraphic units and a regional F4 monocline can be traced across the fault line with minimal displacement. To the southwest, the fault crosses but does not displace a distinct positive gravity anomaly associated with the Upper Dalradian pelitic rocks of S Achill and SW Corraun (pers. comm. Prof. T. Murphy). If the authors had attempted to continue the fault to the southwest, they would have found that its course into the island of Achill Beg coincides with a minor fault with an apparent dextral displacement of about 30 m. To the northeast of Corraun Hill, the fault has been extrapolated beneath a very extensive area with no exposure. The implications of recognising the Corraun fault as a

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