444 STRATIGRAPHICAL GEOLOGY 



Feet. 



Upper Calcareous Grit, shales, and grit. Cardioceras 



alternant, Belemnites nitidus, and Goniomya lilerata 30 to 40 



Upper limestones, coral rags, and oolites with Peri- 

 sphinctes plicatilis, Bourguetia striata, Trigonia pcr- 

 lata, Cidaris florigemma, Thamnastrca concinna, and 

 other corals . . . . . . . . 40 to 50 



Middle Calcareous Grit with Perispkinctes plicatilis . 40 to 80 



Lower limestones with Cardioceras cordatum and Ni/cleo- 



lites scutatus ........ 30 to 60 



Graystones, coarse, gritty, and cherty limestones witli 

 Aspidoceras yoliathus, Gervillia aviculoides, and 

 Rhynclionella Thurmanni . . . . . . 25 to 40 



o 5 

 tsj a, 



Lower Calcareous Grit, gritty limestones, and soft cal- 

 careous sandstone with Aspidoceras perarmatuin, 

 Rhynclionella Thurmanni, and Collyritcs bicordatvs . 50 to 130 



From 215 to 400 



3. The Kimeridge Clay. This formation takes its name 

 from Kimeridge Bay near St. Alban's Head in Dorset, where it has 

 a thickness of no less than 1000 feet and is divisible into - 



Feet. 



The Upper Kimeridge or zone of Perisphinctes biplcx . . . 600 

 The Lower Kimeridge or zone of Cardioceras alternans . . . 400 



In Dorset and Wilts the lower part consist* of dark clays and 

 shales with layers of septaria and cement-stones ; these beds yield 

 Cardioceras alternans, Perisphinctes mutabilis, Ostrea deltoidea, and 

 Rhynclionella inconstans. The upper beds are black bituminous 

 shales and grey papery shales, often full of broken and compressed 

 shells ; they yield Perisphinctes biplex, Protocardia striatula, Thracia, 

 depressa, Exogyra virgula, and Lucina minuscula. Reptilian bones 

 occur throughout the Kimeridge Clay, and among them are remains 

 of two species of Pterodactyles. 



There are few good inland sections of the Kimeridge Clay, and 

 its thickness is greatly diminished in passing through Dorset and 

 Wiltshire, for at Swindon it is estimated to be only about 300 

 feet ; and in Berkshire and Oxfordshire it is not much more than 

 100 feet, but. is still divisible into the two zones. 



Near Aylesbury the Kimeridge Clay is not more than 100 feet 

 thick, and merges upward into a sandy clay containing Portlandian 

 fossils (the Hartwell Clay). Near Leighton Buzzard, and thence 

 through Bedfordshire, most of the Kimeridge Clay is concealed by 

 the Cretaceous sands, and does not emerge again till we reach the 

 neighbourhood of Papworth in Cambridgeshire, where it appears 

 above the Ampthill Clay, and runs in a narrow belt by Knapwell, 

 Boxworth, and Cottenham to the Fens. 



Near Ely this clay has been largely dug, and both zones (upper 



