390 THE WONDERS OF GEOLOGY. Lect. IV- 



limestones, full of marine shells ; upon these is the dirt-bed 

 with fossil trees (2) ; this is covered by cream-coloured 

 calcareous stone in thin undulated laminae, locally termed 

 "soft-bur?*" (1) ; and above are shales and narrow bands 

 of limestone, belonging to the lower series of the Purbeck. 

 The dirt-bed has been discovered by Dr. Buckland 

 near Thame in Oxfordshire ; and by Dr. Fitton, in the 

 Vale of Wardour. It also occurs at Swindon in Wiltshire 

 on the top of the Portland oolite, where fossil conife- 

 rous wood is found in abundance, and a few examples of 

 Mantellia have been obtained. Between Stone and Hart- 

 well in Buckinghamshire, a seam of carbonaceous earth 

 occupies the geological position of the Portland dirt-bed, 

 and is covered by cream-coloured marls and limestones 

 resembling the Cap, in which wings of insects, and 

 leaflets of Wealden ferns, with remains of very small 

 fishes, have been discovered.* 



From what has been stated, it is evident, that after the 

 marine strata forming the base of the Isle of Portland were 

 deposited at the bottom of a deep sea, and had become 

 consolidated, the bed of that ocean was elevated above the 

 level of the waters, and constituted an island, or archi- 

 pelago, covered with pine forests, and cycadeous plants. 

 How long this new country existed, cannot be ascertained, 

 but that it flourished for a considerable period is certain, 

 from the number and magnitude of the petrified trees. f 



* See History of Fossil Insects in the secondary rocks of England, 

 by the Rev. P. B. Brodie. 



■\ Modem submerged Forest. — An interesting modern example of 

 the subsidence of a considerable tract of country clothed with forests, 

 the trees remaining erect, although submerged beneath a river which 

 still flows over them, is described by a late American writer, and will 

 serve to illustrate the remarks in the text. The whole district, from 

 the Rocky Mountains on the east and the Pacific Ocean on the west, 

 and from Queen Charlotte's Island on the north to California on the 

 south, presents one vast tract of volcanic formation. Basalt — both 



