Ch. XX.] 



CHANGES OF MEDIUM. — PURBECK BEDS. 



299 



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 tvhich the strata ivere formed, 

 from the Portland Stone up to the Lower Greensand inclusive, in the 

 southeast of England {beginning with the lowest). 



1. Marine 



2. Freshwater 

 Land 



Freshwater 

 Land 



Freshwater 

 Land (Dirt-bed) ' 

 Freshwater 

 Land 

 Brackish 

 Freshwater 



Portland Stone. 



► Lower Purbeck. 



Marine 



Freshwater 



Marine 



Brackish 



Marine 



Brackish 



Freshwater 



Freshwater 



Freshwater 



Brackish 



Freshwater 



Freshwater 



Marine 



• Middle Purbeck. 



Upper Purbeck. 



Hastings Sands. 



Wealden Clay. 

 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, or from 

 these again to a state of land, which have occurred in this part of Eng- 

 land between the Oolitic and Cretaceous periods. That there have been 

 at least four changes in the species of testacea during the deposition of 

 the Wealden and Purbeck beds, seems to follow from the observations 

 recently made by Prof. Forbes, so that, should we hereafter find the 

 signs of many more alternate occupations of the same area by different 

 elements, it is no more than we might expect. Even during a small part 

 of a zoological period, not sufficient to allow time for many species to die 

 out, we find that the same area has been laid dry, and then submerged, 

 and then again laid dry, as in the deltas of the Po and Ganges, the his- 

 tory of which has been brought to light by Artesian borings.* We also 

 know that similar revolutions have occurred within the present century 

 (1819) in the delta of the Indus in Cutch,f where land has been laid 

 permanently under the waters both of the river and sea, without its soil 

 or shrubs having been swept away. Even, independently of any vertical 

 movements of the ground, we see in the principal deltas, such as that of 

 the Mississippi, that the sea extends its salt waters annually for many 

 months over considerable spaces which, at other seasons, are occupied by 

 the river during its inundations. 



It will be observed that the division of the Purbecks into upper, middle, 

 and lower has been made by Prof. Forbes, strictly on the principle of the 

 entire distinctness of the species of organic remains which they include. 

 The lines of demarcation are not lines of disturbance, nor indicated by 

 any striking physical characters or mineral changes. The features which 

 attract the eye in the Purbecks, such as the dirt-beds, the dislocated 

 Btrata at Lulworth, and the Cinder-bed, do not indicate any breaks in the 



* See Principles of Geol. 9th ed. pp. 255-275. 



f Ibid. p. 460 



