227 



defined as *'argiles coinposees d'alternances de mames grises 

 ou bleues avec intercallatious de calcaires blancs grisatres ou 

 jauiiatres avec Ostrea virgula, Ostrea spiralis, Pterocera ponti. 

 Ammonites nucleus, Trigonia supra-jwrensis, &c." These strata 

 have a thickness of about 80 metres. 



M. Thoemann makes this group his second division, or mames 

 Kimmeridiennes, and which he defines as "marnes jaunatres et 

 calcaires marno-compactes, grumeleux, avec exogyres et nom- 

 breux fossiles, a I'etat de moule interieur," and in his later 

 letters on the Jura considers the thickness of these marnes to 

 be 35 metres. 



During the formation of the Great Western Railway by 

 Wotton Bassett a section of the Kimmeridge clay was made, 

 and from this I obtained some of its characteristic fossils, as 

 Ammonites decipiens, Amm. m,utabilis, Pleurotomaria reticulata, 

 Ostrea virgula, Ostrea deltoidea, &c. This formation is, however, 

 feebly exposed in Wiltshire. Near Tarringdon, Berks, it 

 consists of a dark blue or olive-coloured clay, with sandy or 

 calcareous bands containing fossils, and is overlain by the 

 Sponge gravel beds of the Lower Greensand ; and at Culham 

 by the blue laminated clay of the Gault, containing Ammonites 

 interruptus. At Heading-ton Hill it is seen in position resting 

 upon the Coralline OoUte, and here contains Exogyra virgula, 

 Ostrea deltoidea, Rhynckonella inconstans. Ammonites hiplex, with 

 large bones of Dinosaurians belonging to the genera Pliosaurus, 

 Ichthyosaurus and Steneosaurus. 



The fine section on the Dorset coast, between St. Alban's 

 Head on the east, and Gad Cliff on the west, embracing its 

 type locality — Kimmeridge Bay, affords a magnificent exposure 

 of this formation, which is best seen by sailing along the coast ; 

 near the shore it forms the lower portion of the cliffs, and 

 consists of dai'k blue bituminous clay, overlain at Svs^yre Head 

 by Portland sand and stone. 



POKTLANDIEN = PoKTLAND OoLITE. 



M. Martin defines these ten-ains as — 



( n Calcaires mameux et mamo-compactes a Trigmtia 

 PoETLAXDlEN. Bdonknsis et Pinna supra-Jurensis. 



\ h Calcaires marno-compactes a Amm. gigaa. 



