208 PEOCEEDHiTGS OF THE GEOLOGICAL SOCIETY. [Feb. 8, 



racters, and evidence of having been deposited under widely dis- 

 similar conditions. Thus marine formations are found, not only to 

 exhibit all the variations consequent on changes from littoral to 

 deep-sea conditions, but even to pass into or alternate with fresh- 

 water and terrestrial strata and vice versa. So far have discoveries 

 of this kind already advanced, that we shall not .be going too far in 

 stating that, in the case of the Mesozoic rocks at least, there is no 

 great thickness of marine strata with the terrestrial and freshwater 

 equivalents of which we are altogether unacquainted. 



While treating of the peculiarities of the fauna of the period 

 which lies upon the confines of the Palaeozoic and Mesozoic epochs, 

 Professor Huxley has been led to remark that it will ultimately be 

 necessary for geologists to establish two parallel but distinct schemes 

 of classification for strata of different origin, since the breaks 

 between the marine systems do not correspond with those between 

 the freshwater and terrestrial. It is nevertheless clear that the 

 occurrence of terrestrial and freshwater fossils in marine strata 

 (into which they have been carried down), the existence of certain 

 organized beings (as some fish) which live indifferently either in 

 salt or fresh water, the circumstance of the gradual passage between 

 or alternation of freshwater and marine strata, together with the 

 facts of their sequence and the nature of their physical relations, 

 will afford data for correlating, with more or less accuracy, the two 

 schemes of classification. Regarded from this point of view those 

 strata which are of Jluvio-marine origin, and yield at the same time 

 marine, freshwater, and terrestrial fossils, are of special interest and 

 value to the geologist. One of the most remarkable formations of 

 this character I propose to describe in the present paper. 



The first example of a great system of strata of freshwater origin 

 which was clearly recognized by geologists, was the Wealden, the 

 nature of which was demonstrated by Dr. Mantell in 1822. Its 

 exact correlation with the marine formations has, up to the present 

 time, been the subject of frequent controversy ; but recent dis- 

 coveries in Prance, England, Spain, and Germany have furnished us 

 with many of the data requisite for arriving at definite conclusions 

 upon the subject. In a short sketch read before the British 

 Association at its last Meeting I attempted to show how perfectly 

 the inferences with regard to the age of the Wealden-Purbeck to 

 which we are led by the study of its marine fossils, accord with 

 those drawn from an examination of its physical relations to the 

 great marine systems. Prom both kinds of evidence I have en- 

 '"deavoured to establish the following propositions : — 



1. The deposition of the Wealden strata of the south of England 

 commenced before the close of the Oolitic period ; it continued 

 during the whole of the Tithonian, and of the Lower and Middle 

 Neocomian, and only came to an end at the commencement of the 

 Upper l^eocomian. 



2. The deposition of Wealden strata did not extend to the north 

 of France until the latter portion of the period, and the beds in this 

 area are greatly diminished in thickness, while they alternate with 



