Discussion

Abstract

D r D. D. J. A ntia writes: Dr Leggett suggested that the Ludlow-Pridoli boundary can be and has been correlated with certainty in the Lake District, Welsh Borderlands, and Ireland. This suggestion is fallacious because the criteria for the recognition of the boundary in the U.K. have only recently been outlined (e.gKaljo1978), and insufficient palaeontological work has been done to allow these criteria to be applied. If, however, Dr Leggett meant the Ludlow-Downton boundary, correlation is still a problem, since Squirrel1 & White (1978) showed that middle Ludlovian sediments can contain supposedly diagnostic (Siveter 1978) Downtonian ostracods. He also asserted that the black shales contain little or no land derived organic debris. This is surprising since much of the organic debris in modern black shales island derived, and terrestrial algae, cuticular and vascular plants are considered to have inhabited the Lower Palaeozoic land (e.g.Gray & Boucot 1977). He noted only one occurrence of Lower Palaeozoic phosphatic black shales in the Welsh Borderland region, though others have been recorded (e.g. ; Antia 1979), and he suggested that they are analagous to those forming today under oxygen deficient waters off W-facing continental margins. However, he neglected to state whether they formed on an overriding plate between the land and the trench, like the modern Peruvian examples (Burnett 1977) or on a passive Atlantic type margin like the West African examples (Baturin et al. 1972), and failed to comment on their environmental significance with respect to the genesis of the