300 PORTLAND STONE. [Ch. XX. 



distribution of organized beings. " The causes which led to a complete 

 change of life three times during the deposition of the freshwater and 

 brackish strata must," says this naturalist, " be sought for, not simply in 

 either a rapid or a sudden change of their area into land or sea, but in 

 the great lapse of time which intervened between the epochs of deposition 

 at certain periods during their formation." 



Each dirt-bed may, no doubt, be the memorial of many thousand years 

 or centuries, because we find that 2 or 3 feet of vegetable soil is the only 

 monument which many a tropical forest has left of its existence ever 

 since the ground on which it now stands was first covered with its shade. 

 Yet, even if we imagine the fossil soils of the Lower Purbeck to repre- 

 sent as many ages, we need not expect on that account to find them 

 constituting the fines of separation between successive strata character- 

 ized by different zoological types. The preservation of a layer of vege- 

 table soil, when in the act of being submerged, must be regarded as 

 a rare exception to a general rule. It is of so perishable a nature, 

 that it must usually be carried away by the denuding waves or currents 

 of the sea or by a river ; and many Purbeck dirt-beds were probably 

 formed in succession, and annihilated, besides those few which now 

 remain. 



The plants of the Purbeck beds, so far as our knowledge extends at 

 present, consist chiefly of Ferns, Coniferas (fig. 344), and Cycadese (fig. 

 340), without any exogens; the whole more allied 

 to the Oolitic than to the Cretaceous vegetation. F ^L^L 



The vertebrate and invertebrate animals indicate, 

 like the plants, a somewhat nearer relationship to 

 the Oolitic than to the cretaceous period. Mr. 

 Brodie has found the remains of beetles and several 

 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 



Cone of a pine from the 

 rivers. Isle of Purbeck (Fitton). 



Portland Stone and Sand (6, Tab. p. 291). — The 

 Portland stone has already been mentioned as forming in Dorsetshire the 

 foundation on which the freshwater limestone of the Lower Purbeck re- 

 poses (see p. 296). It supplies the well-known building-stone of which 

 St. Paul's and so many of the principal edifices of London are constructed. 

 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 arc 

 almost wholly confined to the southern counties. Corals are rare in 

 them, although one species is found plentifully at Tisbury, Wiltshire, in 

 the Portland sand, converted into flint and chert, the original calcareous 

 matter being replaced by silex (fig. 345). 



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 bituminous 



