46 Prof. BucKLAND and Mr. De la Beche on the Geology of Weymouth, ^c. 



the change to the next state of things was quiet and gradual ; since the trees 

 that he prostrate on this black earth would have been swept away had there 

 been any violent agitation, or sudden irruption of water. 



3rdly, The dry land on which this forest grew became converted to some- 

 thing like an estuary, in which the lowest deposits contain freshwater shells ; 

 these are succeeded by a thick bed of oyster shells, and above the oyster bed are 

 strata containing an admixture of freshwater shells with shells that are marine. 



We have evidence that this formation extended eastward from what is now 

 the coast of Dorset, through the Isle of Wight to the eastern extremity of the 

 weald of Kent; but of the boundaries of this supposed estuary we have not 

 the slightest indication beyond that which is afforded by the existing deposits 

 of Purbeck and Wealden freshwater formations. 



The occurrence of the Purbeck strata reposing on the Portland stone at 

 Lady Down near Tisbury, on the west of Salisbury, in a position directly 

 north of the Isle of Purbeck, at the distance of about thirty miles, renders it 

 probable that the breadth of the estuary in this part extended over the inter- 

 mediate portions of Dorset and Wilts, which are now covered up with chalk. 



4thly, We have a return of the sea over our estuary ; and in this sea an 

 accumulation of the successive and thick marine deposits which constitute the 

 greensand and chalk formations. 



5ih\y, We have in the Isle of Wight a reappearance of freshwater deposits 

 mixed and alternating with others that are marine, through the next great 

 periods of the tertiary formations. 



6thly, All these deposits appear to have been succeeded by a tremendous 

 catastrophe, producing elevations, depressions, and contortions of the strata ; 

 and intersecting them with enormous faults. 



7thly, These movements of the land have been succeeded by inundations, 

 competent to excavate the valleys of denudation, and partially to overspread 

 the country with diluvial gravel. 



8thly, This denudation has been followed by a state of tranquillity, which 

 has remained undisturbed to the present hour. 



