§ 12. THE OXFORD OOLITE. 499 



This deposit is in some places 300 feet in thickness, but 

 thins out to a very inconsiderable layer in some localities ; 

 and in the northern counties of England is altogether 

 wanting. 



Around Hartwell in the vale of Aylesbury, in Bucking- 

 hamshire, this clay is largely developed, and abounds in 

 organic remains of great beauty and interest. I know of 

 no locality which has yielded such splendid specimens of 

 Ammonites, Rostellarise, Pleurotomariae, Pernse, &c. ; 

 the nacreous or pearly coat of the shells of the ammonites 

 is often as perfect and splendid with iridescent colours as 

 in a polished recent nautilus shell. Many of the shells, of 

 which casts only occur in the Portland rock above, are 

 found preserved entire in the clay. A flat oyster of a 

 deltoids form (Ostrea deltoidea) is very abundant, and is 

 generally considered as peculiar to this clay, but it occurs 

 also in the Oxford clay, which is lower in the series. 



Bones of Ichthyosauri, Plesiosauri, and Cetiosauri, are 

 occasionally found : and scales, teeth, and other remains 

 of fishes, among which are mandibles of the Chimaeroids.* 



12. The Oxford Oolite. — The Kimmeridge clay rests 

 on beds of coralline limestones, provincially termed Coral- 

 rag, many of which are literally petrified coral-reefs, 

 consisting of coarse limestone, composed of madrepores, 

 astrese, caryophylliaB, and other stony corals, having the 

 interstices filled up with shells, echinoderms, sand, and 

 pebbles ; the whole is more or less consolidated by calcareous, 

 and in some instances by siliceous, infiltrations. So obvious 

 is the origin of these rocks, that the most incurious observer 

 who travels through the districts where these deposits pre- 

 vail, cannot fail to remark the blocks of coral which every - 



* The extensive Museum of Dr. Lee of Hartwell contains a fine 

 collection of fossils from the neighbourhood, to which strangers are 

 allowed free access by the learned and liberal proprietor. 



VOL. II. L L 



