114 THE LIASSIG PERIODS. CHAP. 



The non-ammonitiferous beds below these, but above the West- 

 bury shales, contain in greater abundance than elsewhere Ostrea 

 liassica, and have been called the Ostrea beds. 



Following the same idea, we may call the lowest (Westbury) 

 group the Avicula group, from the frequency in it of A. contorta, 

 a shell which is still more frequent in foreign triassic beds. 



Following out the method of marking successive deposits by 

 zones of successive life, and keeping to one of the most characteristic 

 groups (ammonites), we find in the south of England, and partially 

 in the midland district, near Oxford, the following stages (of lower 

 lias) above those noted at Saltford : 



k. Shales usually dark, with Ammonites raricostatus. 



i. Shales usually dark, with Ammonites oxynotus. 



h. Shales and included bands of grey limestone, with Ammonites obtusus. 



g. Shales and limestones, with Ammonites Turneri. 



Lima gigantea occurs in g. 

 Gryphsea obliqua g to k. 

 Cardinia ovalis in g. 

 Pentacrinus tuberculatus in g. 

 Pleurotomaria anglica in h. 

 Cardinia Listeri in k. 



Hippopodium ponderosum in k. 

 Unicardium cardioides in k. 

 Rhynchonella variabilis in k. 

 (k is called the Hippopodium bed by 

 Buckman.) 



In these beds Cardinia Listeri and other species of the genus 

 occur pretty frequently, though not uniformly ; and following the 

 plan of grouping by conchifera, we may apply to them all the title 

 of Cardinia beds, marking the zones by the most characteristic 

 ammonites. These beds are best seen in detail near Cheltenham ; 

 but they are completely traced in mass across Warwickshire and 

 Northamptonshire, between the lima beds and the next great group 

 now to be noticed, viz. the marlstone or middle lias group. 



MARLSTONE GROUP. 



The middle lias in the midland counties agrees better with the 

 character of that deposit as it occurs in Yorkshire, than with the 

 less rocky and more argillaceous series of Dorsetshire. In Gloucester- 

 shire the upper part is a solid ferruginous stone, partly calcareous ; 

 the lower part a series of brown, yellow, or grey sands. These are 

 variable, and often thin. There is, in fact, little else in Warwick- 

 shire, Oxfordshire, and Northamptonshire but the usually ferru- 

 ginous rock, called * marlstone'' by William Smith. This rock can 



