ZONE OF AVICULA CONTORTA. 5 



The question where does the Lias formation commence is easier asked than 

 answered, for between the uppermost beds of the grey marls of the Keuper and the 

 lowest true beds of the Lias is a remarkable assemblage of strata, which I long ago 

 described as the " Avicula contorta beds," from that shell forming their leading fossil. 

 Typical sections of the Contorta series are exposed at Garden cliff, Aust cliff, Penarth, 

 and Watchet on the Severn shores ; at "Weston, Keynsham, Willsbridge, and Salford, 

 near Bath, and at Puriton, Uphill, and Wells in Somersetshire, as well as in many other 

 localities. In the upper part of the series are dark-grey shales interstratified with bands 

 of limestone containing Avicula contorta, Cardium Bh<jeticum, Pecten Valoniensis, and 

 Schizodus elongatus ; and in the lower part the Bone-bed, consisting of hard dark 

 siliceous grit, charged with bones, spines, teeth, and scales of Fishes belonging to the 

 genera Nemacanthus, Acrodus, Sargodon, Hybodus, and Ceratodus, with bones of Reptilia 

 belonging to the genera Ichthyosaurm and Plesiosaurus, and the teeth of a small 

 Mammal, Mlcrolestes antiquus, Plieninger. 



As no sketch of the Lias formation can be said to be complete without its relation 

 to the Trias below and the Jurassic rocks above being clearly defined, I shall briefly 

 describe some type sections of the Avicula contorta beds in which the relation of these to 

 the base of the Lias is very well shown. 



Garden Cliff, near Westbury-on-Severn. 



I made a detailed section of the Avicula contorta beds of Garden Cliff, near West- 

 bury-on-Severn, many years ago,^ as it affords one of the most typical profiles of the zone 

 known to me, and that best situated for study, and 1 recommend its careful examination 

 to all who wish to know this series, as each stratum emerges in succession by the river 

 bank, and can be measured and searched for its fossils, as it rises in the cliff at a low 

 angle. The Rev. W. D. Conybeare" gave a section of this, which he called " Westbury 

 Cliff, on the west bank of the Severn," for the purpose of illustrating the longer beds of 

 the Lias formation, which reads as follows : — 



White Lias ...... 



Blue shale passing into Marlstoue .... 



Black shale with iron-shot fissures 



Green siliceous grit, highly micaceous, and containing abundant bones 



well known here and at Aust by the name of the Bone-bed . 

 Black shale 

 Green grit .... 



Black shale 



Greenish Marlstone decomposing into balls 



Red marl of the new red sandstone formation 



1 'Quart. Jour. Geol. See.,' vol. xvi, p. ."378, 1860. 



2 • Geology of England and Wales,' p. 263, 1822. 



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