400 



SPECIAL GEOLOGY. 



the changing conditions under which the strata were formed 

 from the lower greensand to the Portland oolite inclusive, 

 on the south-east coast of England : 



Lower Purbeck 



Portland Stone 



The causes, says Professor Porbes, " that led to a com- 

 plete change of life, three times, during the deposition of 

 these freshwater and brackish strata, must be sought for not 

 simply in either a rapid or a sudden change of their area 

 into land or sea, but in the great lapse of time which 

 intervened between the epochs of the deposition, at certain 

 periods during their formation." It is interesting to find 

 in the Purbeck beds, changes of condition of a similar cha- 

 racter to those which we have described in the eocene strata 

 of the north-west coast of the Isle of Wight,* where fresh- 

 water, brackish, and marine beds in like manner succeed 

 each other, and are characterised by corresponding changes 

 in their organic contents changes which must have re- 

 quired long periods of time for their fulfilment. 



WEALDEF STRATA or THE ISLE or WIGHT. The 

 wealden beds are extensively developed at the south side of 

 the island, and constitute the foundation-rock of Sandown, 

 Brook, and Brixton Bays. Pig. 281 is a coast section of 

 the Isle of Wight from Blackgang to Afton Down, showing 

 the relative position of the wealden and lower greensand* 

 with the western anticlinal axis of the island, which passes 

 through Brook Point. The beds are observed to dip to the 

 east and west at the point. This locality is likewise 

 remarkable for the fossil forest of the wealden, which lies 

 prostrate on the strand at low-water mark. The trunks of 



* Dr. Wright : Annals of Nat. Hist., vol. vii. 



