ZONE OF AVICULA CONTORTA. 9 



Aust Cliff. 



If we proceed from Garden Cliff down the Severn, the next section of the Avicula 

 contorta beds is met with at Aust Cliff, so long famous for its Bone-bed and the large 

 number of Ceraiodus teeth which from time to time have been collected therefrom. 



My old friend, the late Mr. Willliam Sanders, F.R.S., of Bristol, many years ago 

 carefully measured this section, a matter of much difficulty from the mural character of 

 the escarpment, and the result of his labours was published by Sir Henry De la Beche, 

 in his valuable memoir on the geology of the South-west of England, 1 Buckland and' 

 Conybeare having previously published a section of the cliff in their memoir on the 

 south-western coal-district of England. 2 



In the upper part of the section are found about three feet of grey argillaceous Lias 

 Limestone, containing Aegoceras angulation, Schloth., Lima gigantea, Sow., Lima anti- 

 quata, Sow., and Modiola Hittana, Sow., representing the lower beds of the Lima-series. 

 Below these are nine beds consisting of grey marls and argillaceous limestone repre- 

 senting the zone of Aegoceras planorbis. The lowest limestone bed of the series 

 contains scales of Fishes, elytra of Insects, with Modiola minima and Terebratula ; 

 this rests upon eight feet of grey, light-coloured marls, with nodular limestone, the 

 equivalent of the Ostrea-beds and White Lias series, of the strata numbered two to 

 six inclusive in the detailed section of Garden Cliff (p. 6). The Cotham Marble caps 

 the marly beds, and this well-known singular band forms the base of the Ostrea-series. 

 The gap in the section is intended to represent a space of thirteen feet which could not 

 be satisfactorily examined. Below this space five feet of thinly laminated black shales 

 are exposed, and beneath is a five-inch band of hard grey limestone, containing scales of 

 Fish, Pecten Valoniensis, Placunopsis alpina, Pleurop/iorus elongatus, representing the 

 upper Pecten bed of the Garden Cliff section, &c. ; this band rests upon another bed of 

 black shales, eight feet in thickness, containing seams of shells. Here are found 

 Avicula contorta, Cardium Rhaticum, Pullastra arenicola, Axinus, Anomia, &c. The 

 fossils are very numerous, but much compressed, and determined with difficulty. The 

 Contorta-shales rest upon a second or lower Pecten-bed, consisting of a hard grey shelly 

 limestone, eight inches thick, containing Avicula contorta, Cardium Rhaticum, Pullastra, 

 Axinus, Anomia, &c, in fact all the same shells that are found in the black shales above. 

 Beneath the limestone band is another bed of black shales, four feet thick, intersected 

 by thin, inconstant, indurated bands, containing Fishes' scales, &c. The shales rest 

 upon the true Bone-bed, which is here a most remarkable band of dark-grey, crystalline, 

 calcareo-siliceous rock, containing nodules of marl, masses of dark coprolitic matter, 

 bones of Saurians, teeth of Ceratodus and other Fishes, in fine preservation. Its thickness 

 varies from two to eight inches, and it rests upon a thin band of dark shales, to which 

 succeeds a yard of grey sandy marls, containing hard concretionary nodules ; beneath 

 are bands of hard sandy marl, resting upon eight feet of pale-grey arenaceous marls, 



1 ' Memoirs of the Geological Survey of Great Britain,' vol. i, p. 253, 1846. 

 3 ' Geological Transactions,' 2nd series, vol. i, pi. 37, 1824. 



2 



