Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/83

 the States, which is bordered by Tertiary strata of a like character with their European equivalents.

It is true there have been elevations of the Cretaceous and Tertiary strata during the Tertiary period far greater than the depths first mentioned ; but it has been in mountain-chains which have little affected the great plains of continental land. In the same way there may have been partial elevations in the bed of the Postcretaceous Atlantic ; but there is nothing to indicate that it has ever been entirely raised. I think, therefore, that the hypothesis with regard to the continuity of that sea-bed from the period of the Chalk to the present period is one of high probability.

If such a northern land barrier as that which I have alluded to existed at the period of the Chalk, and that barrier was submerged during the early part of the Tertiary period, it would (taken in conjunction with the very different conditions of depth under which the Chalk and Lower Tertiaries were formed) go far to account for the great break in the fauna of the two periods. Some years since I had occasion to show on other grounds that the Thanet Sands, which repose on the Chalk in the south-east of England, exhibited a fauna essentially of temperate or cold latitudes, and I inferred the inset of currents from the north. As those remarks bear upon the present question, I will quote some of the passages in the paper to which I refer *.

" In viewing the London Tertiaries as a group, and comparing them directly with the underlying Chalk, it is to be observed that we are not comparing like terms of the two periods. That a great and essential difference existed between these periods must be admitted ; but it is a question how far that difference is widened by the comparison being instituted between the deep and shallow sea deposits, instead of between strata deposited under like conditions during those two periods The adaptation of this area at the Thanet-Sands period to the existence of the numerous shallow- water burrowing Lamellibranchiates, whatever the duration of the intervening time, would necessarily unfit it for the deeper-sea Cephalopoda, Brachiopoda, and other families which prevail in our Cretaceous series.

"We have therefore, in viewing the Tertiary strata in relation to the underlying Chalk, to take into consideration that the existence of certain classes of fossils in the former of necessity implies the non-existence of other classes found in the latter deposit — and


 * Quart. Journ. Geol. Soc. vol. x. p. 443, Nov. 1854.