10 THE LIAS AMMONITES. 



having at their base a nodular band of similar marls, passing into a thick bed of sandy 

 marlstone, six feet six inches thick, and forming the base of the Avicula coniorta series, 

 which rests on nodular greenish marls, thick-bedded and red-striped, fourteen feet thick, 

 and next a thick mass of marls fifty-two feet thick, having a conchoidal fracture ; then 

 follow the gypsiferous series, containing fibrous gypsum, in string-like lines, for twenty-, 

 five feet six inches, having at the base twenty feet of red sandy marls, the whole resting 

 upon highly inclined strata of the lower portion of the Carboniferous Limestone, which 

 here forms the bed of the Severn. 



Penauth Clifi?. 



At Penarth Head, near Cardiff, a magnificent section is exposed, showing the Lower 

 Lias and Avicula contorta beds resting on the variously coloured marls of the Keuper. 

 The lowest half of the cliff is a roll of the same beds which occur at Garden Cliff, forty- 

 four miles higher up the Severn. The upper part of the Head is composed of alternate 

 beds of shale and limestone with Arietites Turneri, Cardina ovalis, Grypliaea arcuafa, and 

 Pentacrinus tuberculatus, fifteen feet thick, which overlie the Zone of Arietites BucJdandi, 

 consisting of alternate beds of limestone and shale, which attain from fifty to sixty feet in 

 thickness ; and contain Lima antiqua. Sow., Lima gigantea. Sow., Lima punctata. Sow., 

 Lima pectinoides. Sow., Cardinia hylrida, Stuch., JJnicardium cardioides, Phil., with 

 fragments of Arietites Bucldandi. Beneath the Lima-series are beds of laminated 

 clay containing Aegoceras planorhis. Sow., with alternate strata of clay and limestone, and 

 a profusion of Ostrea liassica on the surface of the slabs. They attain a thickness of ten 

 feet, and are underlain by a bed of stiff clay containing Ostrea liassica and Modiola minima. 

 The Avicula-contorta-hedi^ form an important feature in the cliflf, consisting as they 

 do of blackish shales interposed between the light-coloured marls of the Keuper below, 

 and the light-coloured shales and limestone of the Planorbis-beds above ; beneath are 

 dark marly clays with impressions of teeth, scales, and shells that lie compressed in seams 

 of clay, resting on a calcareous rocky band containing Pecten Valoniensis, Defr., closely 

 pressed together, and badly preserved. Beneath this Upper Pecten-bed are several 

 feet of dark shales, resting on a second stony band with Pecten Valoniensis, ai^A this lower 

 Pecten-band is underlain by shales and layers of ripple-marked micaceous sandstone, 

 containing Avicula contorta and Pullastra arenicola, Strick. The Bone-bed occm's in 

 the section, but is feebly represented, and often with difficulty discovered, as its contents 

 are sparse, and often much comminuted. Two feet of Black Shales separate the sand- 

 stone from the Keuper. 



In the county of Somerset there are several exposures of the Contorta-beds laid open 

 either by railway-cuttings or by the action of the sea along the coast ; to the former 

 belong the Uphill Cutting near Weston-super-Mare. As I watched this exposure 

 during its entire progress, and have studied it several times since, I give the details, for it 

 is very typical of the others.^ 



1 ' Quart. Journ. Geol. Soc.,' vol. xvi, p. 382, I860. 



