392 



CHANGES OF MEDIUM— PUKBECK BEDS. 



[Oh. XX. 



aii^le in an opposite direction — a beautiful illustration of a change 

 in the position of beds originally horizontal (see fig. 379). Traces 

 of the dirt-bed have also been observed by Mr. Fisher, at Ridgway ; by 

 Dr. Buckland, about two miles north of Thame, in Oxfordshire ; and 

 by Dr. Fitton, in the cliffs in the Boulonnois, on the French coast : 

 but, as might be expected, this freshwater deposit is of limited extent 

 when compared to most marine formations. 



From the facts above described we may infer, first, that those beds 

 of the Upper Oolite, called " the Portland," which are mil of marine 

 shells, were overspread with fluviatile mud, which became dry land, 

 and covered by a forest, throughout a portion of the space now occu- 

 pied by the south of England, the climate being such as to admit the 

 growth of the Zamia and Cycas. 2 dry. This land at length sank down 

 and was submerged with its forests beneath a body of fresh water, 

 from which sediment was thrown down enveloping fluviatile shells. 

 3dly. The regular and uniform preservation of this thin bed of black 

 earth over a distance of many miles, shows that the change from dry 

 land to the state of a freshwater lake or estuary, was not accompanied 

 by any violent denudation, or rush of water, since the loose black 

 earth, together with the trees which lay prostrate on its surface, must 

 inevitably have been swept away had any such violent catastrophe 

 taken place. 



The dirt-bed has been described above in its most simple form, but 

 in some sections the appearances are more complicated. The forest 

 of the dirt-bed was not everywhere the first vegetation which grew in 

 this region. Two other beds of carbonaceous clay, one of them con- 

 taining Cycadece, in an upright position, have been found below it, and 

 one above it, which implies other oscillations in the level of the same 

 ground, and its alternate occupation by land and water more than 

 once. 



Table showing the changes of Medium in which the strata were 

 formed, from ike Portland Stone up to the Lower Greensand in- 

 clusive, in the southeast of England {beginning with the lowest). 



1. Marine 



Portland Stone. 



3. Marine 



1 



2. Freshwater 





Freshwater 



1 



Land 





Marine 





Freshwater 





Brackish 



Y Middle Purbeck. 



Land 





Marine 



1 



Freshwater 

 Land (Dirt-bed) 



- Lower Purbeck. 



Brackish 



Freshwater 



1 

 J 



Freshwater 





4. Freshwater 



Upper Purbeck. 



Land 





5. Freshwater 



) 



Brackish 





Brackish 



]• HastiDgs Sands. 



Freshwater 





Freshwater 



) 







6. Freshwater 



Wealden Clay. 







7. Marine 



Lower Greensand. 



The annexed tabular view will enable the reader to take in at a 

 glance the successive changes from sea to river, and from river to sea, 



