394 



FOSSILS OF THE PORTLAND STONE. 



[Ch. XX. 



Fig. 380. 



denuding waves or currents of the sea, or by a river ; and many Pur- 

 beck dirt-beds were probably formed in succession and annihilated, 

 besides those few which now remain. 



The plants of the Purbeek beds, so far as our 

 knowledge extends at present, consists chiefly of 

 Ferns, Coniferae (fig. 380), and Cycadege (fig. 376), 

 without any angiosperms ; the whole more allied 

 to the Oolitic than to the Cretaceous vegetation. 

 The vertebrate and invertebrate animals indicate, 

 like the plants, a somewhat nearer relationship to 

 the Oolitic than to the Cretaceous period. Mr. 

 Gone of a pine from the Brodie has found the remains of beetles and sev- 

 isieofPurbeck. (Fitton.) Qm \ insects of the homopterous and trichopterous 

 orders, some of which now live on plants, while 

 others are of such forms as hover over the surface of our present 

 rivers. 



Portland Oolite and Sand (5, Tab., p. 375). — The Portland oolite 

 has already been mentioned as forming in Dorsetshire the founda- 

 tion on which the freshwater limestone of the Lower Purbeck reposes 

 (see p. 389). It supplies the well-known building-stone of which St. 

 Paul's and so many of the principal edifices of London are construct- 

 ed. This upper member rests on a dense bed of sand, called the 

 Portland sand, containing for the most part similar marine fossils, 

 below which is the Kimmeridge clay. In England these Upper 

 Oolite formations are almost wholly confined to the southern coun- 

 ties. Corals are rare in them, although one species is found plenti- 

 fully at Tisbury, "Wiltshire, in the Portland sand, converted into flint 

 and chert, the original calcareous matter being replaced by silex 

 (fig. 381). 



Fig. SSI. 



Ieasircea oblong a, M. Edw. and J. Hairne. 



As seen on a polished slab of chert from 



the Portland Sand, Tisbury. 



Fiff. 3S2. 



Trigonia gibbosa. i nat. size. 



a. The hinge. 



Portland Stone, Tisbury. 



The Kimmeridge clay consists, in great part, of a bituminous shale, 

 sometimes forming an impure coal, several hundred feet in thickness 

 In some places in Wiltshire it much resembles peat ; and the bitu- 



