Abstract

Harper and Brenchley (preceding paper) challenge my conclusion that Silurian sediments in southwest central Ireland were deposited in an oceanic trench related to the northwest margin of the Proto-Atlantic Ocean. In my paper on the geosynclinal development of the British Isles (Ziegler 1970), I suggested that the Ordovician and Silurian greywackes of the Southern Uplands, and their equivalents along strike in Ireland, represent an ocean floor. Harper and Brenchley admit that the problems of interpreting the central Irish areas are severe. The strong deformation has fragmented the stratigraphic sections, the rare shelly assemblages that do occur seem to have been transported or are nekto-planktonic in nature, and very little modern work has been devoted to these areas. Only in the Slieve Bernagh-Cratloe Hills area has a stratigraphic column been re­constructed (Weir 1962; Rickards & Archer 1969) so that the arguments of Harper and Brenchley based on the regional extent of conglomerate lenses and shelly assemblages must be tentative in nature. As an example of the fragmentary nature of the stratigraphic record, the largest of the inliers, the Devilsbit Mountain district, is entirely underlain by the part of a single stratigraphic unit that was deposited during the uppermost two Wenlock graptolite zones (Cope 1959; Palmer 1970).

Clearly one cannot sort out the geological problems in Ireland on a basis of parochial considerations alone. The basic framework can only be established in conjunction with the better exposed and more completely studied areas of Scotland and the north of England.

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