CHAP. IX.] JURASSIC PERIOD. 145 



True Purbecks occur in Sussex near Battle, where their 

 thickness is estimated at 400 feet, so that we may suppose 

 them to be continuous from Dorset eastward through 

 Hants and Sussex, but they probably die out in Kent, and 

 are not known in the north of France. 



In the Lower Purbecks of Dorset dirt-beds or carbona- 

 ceous soils are a conspicuous feature, and the presence of 

 rooted stumps of cycadean and coniferous trees proves 

 them to be actually terrestrial surfaces. At the base of the 

 Middle Purbecks is a carbonaceous shale from which twenty- 

 four species of small marsupial mammals, together with 

 the bones of several crocodiles and lizards have been ob- 

 tained. It has been pointed out by Professor E. Forbes l 

 and Mr. C. J. A. Meyer, 2 that the changes from freshwater 

 to marine deposits are abrupt, and that there is no real 

 intermingling of marine and freshwater fossils in the same 

 stratum, but a gradual return from brackish to freshwater 

 conditions ; these facts seeming to indicate lacustrine areas 

 which were subject to occasional and sudden inroads of the 

 sea, and, therefore, in all probability, lakes or lagoons in 

 a silted-up bay or gulf. There is little evidence of direct 

 fluviatile action ; drift wood or plant remains are rarely 

 found except in direct connection with terrestrial surfaces. 

 One of the limestones, containing freshwater shells and 

 nodules of chert, resembles those of the Tertiary lacustrine 

 strata of central France. Lastly, the limestones of the 

 Upper Purbecks being chiefly composed of the shells of the 

 freshwater mollusc Paludina, are such as would be formed 

 in quiet lacustrine waters. 



The Purbecks of Sussex are a much less variable group, 

 and were apparently formed in somewhat deeper water ; 

 they do not include any dirt-beds, but consist chiefly of 

 shales, with two groups of hard grey thin-bedded lime- 



1 "Brit. Assoc. Rep.," Sect. C. } 1850. 



2 " Quart. Journ. Geol. Soc.," vol. xxviii. p. 245. 



L 



