Ancient Geological Changes in England. 301 



•* ] . Tlie Portland limestone, No. 8, containing the remains of 

 none but marine animals and shells, must have been deposited 

 beneath salt water. The species of these shells, it is true, no 

 longer exist ; but of the genera, no one living species is known 

 to inhabit fresh water — all are marine. 



" ^. The mass of the Portland strata must have been raised 

 from the waves, and must have continued to be dry land for a 

 time sufficient for the growth of the trees and Cycadea?, whose 

 remains are still found upon their surface *. 



" 3. But above the soil affording these trees and plants, we 

 now find beds of slaty limestone, — in the Isle of Portland, in 

 Wilts, and Buckinghamshire ; and, in the Isle of Purbeck, be- 

 sides such slaty beds, a considerable thickness of compact lime- 

 stone, full of shells, is so connected with the strata of the Hast- 

 ings sands and Weald clay, as to prove that the whole were 

 deposited continuously. To admit of this, it is obvious, that, 

 cifler the plants and trees had grown and flourished on the top 

 -of the Portland beds, the whole surface of what then was land 

 must have been submerged, to such a depth, as to allow of the 

 accumulation over it of all the Wealden group, which cannot be 

 estimated at less than 700 feet in thickness. And this submer- 

 sion, to all appearance, whether sudden, or, as seems most pro- 

 bable, gradual and slow, was effected tranquilly ; for in many 

 cases the trunks of the petrified trees retain their upright posi- 

 tion, within the substance of the calcareous strata, by which they 

 are now surrounded. 



" 4. The fossils of the beds, thus deposited above the vege- 

 table soil of Portland, are all such as might have been produced 

 in fresh water communicating with the sea. In the waters of 

 this estuary, and of the river of which it may have been the 

 mouth, the aquatic animals must have been nourished, whose 

 remains we now find so profusely throughout the strata of the 

 Wealden. But dry land also must have been near at hand : 

 — 'In fact, its existence at no great distance seems clearly indi- 

 cated, by the remains of the vegetables and amphibia of Tilgate 

 Forest ; some of the former must have grown on the borders of 



• These trees occur in a bed, upwards of a foot thick, of a bituminous clay 

 or soil, named dirt bedj which is interposed between the Portland and Purbeck 

 beds. 



