Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/338

 racters, and evidence of having been deposited under widely dissimilar 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 fluvio-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 discoveries in France, 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 endeavoured 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 Neocomian.

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