1871.] jxtdd ptjiffield foematiox. 207 



Febrfary 8, 1871. 



The following communications were read : — 



1. On the PirifFiELD FoEMATTON. By John W. Jvbj), Esq., F.G.S., 

 of the Geological Survey of England. 



Contents. 



I. Introduction. 

 II. Bibliography of the subject. 



III. Sections in the Isle of Purbeck. 



1. Punfield Cove. 



2. Worborrow Bay. 



3. Mewps Bay. 



4. Lulworth Cove. 



IV. Sections in the Isle of Wight. 



1. Brixton Bay. 



2. Compton Bay. 



3. Sandown Bay. 



V. Sections in the Weald of Sussex, Surrey, and Kent. 



1. Leith Hill, near Guildford. 



2. Hythe. 



VI. Eelations of the Punfield Formation to the Wealden, Neopomian, and 

 Cretaceous of the south of England. 



1. Unconformity between the Cretaceous and Neocomian. 



2. Variation in character of the Cretaceous, in proceeding from 



east to west. 



3. Thinning-out westward of the Neocomian and Wealden. 



4. Relation of the Purbeck to the Wealden. 

 VII. Foreign equivalents of the Punfield Formation. 



1. Urgonien aud Bhodanien of France, Switzerland, &o. 



2. Coal-bearing strata of eastern Spain. 

 VIII. Conclusion. 



I. Introduction. 



The existing scheme of classification of geological formations is in 

 the main founded on the study of marine strata only. The cir- 

 cumstance that rocks of freshwater and terrestrial origin occupy 

 much smaller areas than those of marine character, doubtless in part 

 accounts for this result, which, however, is to a much greater extent 

 attributable to the fact that the fossils of the former class of strata 

 are much less numerous and less strikingly characterized than those 

 of the latter. With the progress of geological research and its 

 gradual extension over wider areas, the necessity for successive 

 enlargements and modifications of our scale of geological periods has 

 from time to time become manifest. The conclusion, so strongly 

 insisted upon by Mr. Darwin and Professor Eamsay, that unconformi- 

 ties between strata indicate the lapse of enormous periods of time, is 

 being constantly confirmed by the discovery, as fresh districts come 

 under investigation, or old ones are more accurately studied, of 

 formations which more or less completely represent these "breaks." 

 Not less important in its bearing upon geological theory is the fact 

 that old and well-known formations, as they are studied at points 

 more and more remote from the typical district, exhibit new cha- 



q2 



