Abstract

Einar Poole writes: I endorse Hallam’s (1990, 1991) views on this subject which are broadly in line with most of those expressed in my 1979 paper. I further agree with him that the relocation of this boundary by Cope’s committee (1980) is a retrograde step that relies on the subjective first entry of the ammonite Psiloceras planorbis, which fossil could well be facies related or geographically controlled. Similarly the Blue Lias is a limestone facies, the diachronous base of which ranges in age from Pre-planorbis Beds in Somerset to the Schotheimia angulata Zone in Warwickshire. The White Lias limestones represent a marginal, shallow water facies of the Pre-planorbis Beds (Poole 1979) which, in places (Stowell Park, Banbury district etc.), are underlain by a thin bed of dark shaly mudstone containing abundant fossils which include Eoguttlina liassica (Strickland), Lenticulina aff. muensteri (Roemer), Nodosana cf. nitida Brand; Diomyopsis intusstriata (Emmerich), ‘Gervillia’ praecursor (Quenstedt), Meleagrinella fallax (Pfflucker), Modiolus aff. langportensis (Richardson & Tutcher), Plicatula sp., Plagiostoma sp. and ostracodes. Most of these fossils also occur in the succeeding White Lias limestones and equivalent Pre-planorbis Beds and some commence their range into younger Jurassic strata here. At this point I differ from Hallam, and the views of George et al. (1969) who recommend the base of Richardson’s (1911) ‘Paper Shale’ in Somerset as the most suitable Triassic-Jurassic boundary. This horizon cannot be traced laterally into the English Midlands and the rest of Great Britain as it contains no diagnostic fossils or features. The lower

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