Jurassic Chronology: III—Some Faunal Horizons in Cornbrash

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Abstract
This communication was commenced and mainly written several years ago: it was intended to form the concluding part of the paper on Watton Cliff (III, 8) following after consideration of Fuller's Earth, Great Oolite, etc., strata. But that paper grew to a length which rendered necessary exclusion of studies of strata later than Inferior Oolite. Now lack of time makes further partition necessary, so that the Cornbrash portion is taken by itself. The Cornbrash has usually been described as a comparatively thin band of limestone-rock stretching continuously across England from the coast of Dorset to the coast of Yorkshire, parting the Oxford Clay above from Forest Marble Clay below. It is implied in such description that the Cornbrash is a synchronous deposit, which took a short time to form, and that it contains a comparatively homogeneous fauna. Is such a view correct, or is the continuity of the Cornbrash deceptive? Is not this band of limestone made up of various heterochronous deposits and how far is its fauna homogeneous? Many years ago, my father noticed the phenomena which form a basis of the present studies—the distribution of dissimilar faunas —in regard to the brachiopods of the Cornbrash. In 1853, speaking on the subject at the Meeting of the British Association for the Advancement of Science (II, 2) he remarks: ‘The author expressed his opinion that certain oolitic Terebratulæ (viz. T. digona, obovata, lagenalis, ornithocephala ) should be considered as forming one species, at the same time admitting that these forms characterized

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