144 



Oolitic Cokals. 



The Oolitic formations will long remain classic ground to 

 English Geologists, as it was during the study of these rocks 

 that Dr, William Smith first obtained the facts which enabled 

 him to "identify strata by organic remains," and thereby to 

 estabKsh a true natural system of Stratigraphical Geology. 



The Oolitic period admits of a sub-division into three groups, 

 — the Lower, Middle, and Upper. Each of these is based on a 

 great argillaceous formation, on which rest minor beds of Sands 

 and Limestones, mostly Oolitic or Pisolitic, in lithological 

 character. The argillaceous formations form broad valleys, for 

 the most part extending diagonally across England, in a direction 

 north-east by south-v/^est. The Sands and Limestones, resting 

 upon the Clays, form low ranges of hills, with an escarpment 

 facing the south-west and overlooking the valley. The base of 

 the Lower Oolites is the Lias ; that of the Middle Oolites, the 

 Oxford Clay ; and the Upper, the Kimmeridge Clay. 



In studying the Oolitic Corals, it is important to keep this 

 classification in view, as not only the groups, but the subordinate 

 members of each, possess specific forms which characterise them. 



Classification of the Oolitic Eokmations. 



Period. Groups. Formations. Thickness. 



- „ ,1 1 TT r^ i-x / Puvbeck beds ... 150 ft. 



i^ortlanu or Upper Uolites, I 



„,,,, ,. , .1 . , { Portland beds ... 170 n 



900 leet thick i 



'. Kimmeridge Clay 600 n 



Oxford or Middle Oolites, ( Coral Rag 180 m 



Oolitic 800 feet thick "( Oxford Clay ... 600 u 



or Jurassic] f Cornbrash 80 h 



Period Cotteswold or Lower Oolites, I Great Oolite ... 130 n 



600 feet thick 1 Fullers Earth .. 130 n 



[ Inferior Oolite . . . 230 m 



f Upper Lias 300 m 



. Lias formation, 1100 feet thick | Middle Lias 200 m 



\ Lower Lias 600 ir 



The Avicula contorta group, lying between the lowest of the 

 Ostrea beds of the zone of Ammonites planoi-bis, and the uppermost 

 strata of the Marls of the Keuper, have of late years formed the 



