Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/85

 such as here occurs ? Can it be that such a group of generic forms, allied to and closely resembling those found in the same zoological province at the present day, had a yet older existence in more northern provinces — that generic forms of temperate regions have travelled from the north, and have been gradually spread further south, giving, when they encroached upon the more southern forms, a more recent aspect to the faunas of such various geological periods than prevailed in those of the same localities when changes in the distribution of land and water brought back for a time the southern forms which had been temporarily displaced ? "

That much of the difference between the fauna of the Chalk and the Lower Tertiaries must be due to the elevation of the old Chalk ocean-bed (by which the deep-sea life was exterminated and a shallower-water fauna introduced) is now evident from the recent deep- sea dredgings. Suppose, for instance, a portion of the present bed of the Atlantic were raised to the level of the sea-bed of the present English channel, whereby the depth of water would be reduced from 12,000 or 15,000 to 100 or 600 feet. The deep-sea fauna would be destroyed, and the fauna and sandy beds of the English coast would succeed it ; and when these were raised, we should have sand and gravelly beds containing a shallow-water fauna overlying calcareous beds with a deep-sea fauna, and there would be but very few, if any, species common to the two deposits.

As old coast-lines and the oceanic currents changed during the Tertiary periods, we may suppose corresponding changes in the fauna of the littoral and laminarian zones, while the deeper-sea fauna (which was not subject to these changes of conditions) may have had a much longer and more permanent existence. Together with the recurring bathymetrical conditions, the lithological character of the sea-bed further influenced the vitality and persistence of species. The Mollusca of the Calcaire grossier of the Paris basin are, according to M. Deshayes, essentially southern in their character and relations. This formation is separated from the Chalk by the London Clay or its equivalents, and the Woolwich series and Thanet Sands, with the fauna of which it has few species in common, whilst, as I have before mentioned, the species of the Lower Eocene beds have a more northern facies. It is not, however, long since MM. Cornet and Briart found under the equivalents of all these English series in Belgium a friable calcareous bed full of fossils, not like those of the overlying Lower Eocene, but resembling, and in many cases identical with, those of the more recent Calcaire grossier. Again, in the Barton Clay, many