360 GEOLOGY OF THE ISLE OF PURBECK. 



of the Megalosaurus, Lepidotus, &c, with stems 

 and fruits of cycadeous plants, presented so striking 

 an analogy, that it led me, on my first discovery of 

 similar fossils in the Tilgate strata, to seek for the 

 equivalent of the latter in the Stonesfield oolite.* 

 In the Isle of Purbeck, the uppermost group of 

 the Oolite only appears; it is denominated "the 

 Portland Oolite" from the island in which the 

 limestone has for centuries been extensively 

 quarried. This group is separated into three 

 divisions ; viz. 1 , the Portland limestones ; 2, the 

 Portland sands ; and, 3, the Kimmeridge clays, so 

 called, from the bay of Kimmeridge, where these 

 beds are well displayed. 



1. Portland limestones. — Oolitic limestone, 

 abounding in trigoniw, ammonites, pernxc, pluro- 

 tomariw, and other marine shells ; layers and 

 nodular masses of chert and flint; total thickness, 

 from 30 to 40 feet. 



2. Portland sands, — Beds of white and green 

 sand and sandstone, with concretionary masses 

 of grit; the lower strata argillaceous ; total thick- 

 ness, about 130 feet. 



3. Kimmeridge clay. — Dark blue clay, with 



' see The Fossils of the South Downs, 1S22, " On the analogy betfreen the 

 organic remains of the Tilgate beds and those of Stonesfield, near Oxford," 

 p. O'J. 



