1857.] BUCKMAN — OOLITES. 127 



Now, if we review the disposition of the masses of this rock, and 

 examine its structure, we shall, I think, conclude that it was not 

 deposited continuously on a uniform sea-bottom, but in separated 

 lumps, — by the peculiar method of working of the coral-animal, 

 to whose labours this bed is mainly due, and under circumstances 

 in all probability similar to those of our modern coral-reefs. This, 

 however, would of itself induce in the then existing sea a phy- 

 sical change of condition, which had its influence on the succeed- 

 ing deposit, namely, the Kimmeridge Clay. The latter, though differ- 

 ing slightly from the Oxford Clay with which it is in contact, as on 

 the road from Blunsdon to Swindon, is marked by a good set of 

 fossils : these at times are the only guide, as the colour of both the 

 Oxford and the Kimmeridge Clay is very variable, and besides, through- 

 out their whole extent, both are remarkable for the quantity of fine 

 crystals of selenite diffused through them ; these very large at Ash- 

 ton Keynes, in the lower beds of the Oxford Clay ; and the Kim- 

 meridge Clay at Swindon is full of beautiful crystalline groups of 

 sulphate of lime. 



12. Kimmeridge Clay. 



Along the line of section, both in its direction to Swindon 

 and Wanborough, this deposit will be seen to succeed the Oxford 

 Clay, of which, indeed, it may, as far as the district under review 

 is concerned, be considered as the upper beds, having the Coral-rag 

 as an occasionally interpolated and local deposit. 



The Kimmeridge Clay occupies a wide range of flat country around 

 the hill on which Swindon stands, and here it is a dark, lead-coloured, 

 unctuous clay, of a shaly consistence when first dug ; the layers 

 of the clay are parted by white nacreous matter, the only remains 

 of thousands of Ammonites and other shells, which are only 

 locally preserved. Every bed of clay that I have examined about 

 New Swindon, where it is largely worked for brick-making, is full 

 of crystals of selenite, and no less remarkable for the quantity of 

 Trigonellites latus, valves of which, all sizes, are best studied here. 



The Kimmeridge Clay at this place may be divided into two 

 stages : — 



2. The upper one of dark shales, in which Trigonellites latus, 

 Exogyra virgula, and Ammonites prevail. 



1 . The lower of lead-coloured and blue clays, full of large masses 

 of Ostrcea deltoidea and abundant sheaths of Belemnites excentricus, 

 Blainville, often of very large size. 



The following list of fossils is by no means complete, and is only 

 offered as evidence of the prevailing facies. 



Reptilia. — A species of Pliosaurus, of immense size ; fine portions 

 of this are in the museum at Cirencester, and in that of the Royal 

 Agricultural College. 



Belemnites excentricus, Blain. Ammonites serratus, M. C. 



Beaumontianus, D'Orb. Goliathus, Ter. Jurass. 



Trigonellites latus, Fitton, Mem. t. 23, t. 195. 



f. n. Sallierianus, id. t. 208. 



