Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/505

1872.] shall presently see, there is reason for the belief that it was not continuous in one direction, but that there were pauses, or even reversions towards the old temperate state, it follows that the two groups of animals would at times overlap, and their remains be intermingled with each other. The frontiers also of each of the geographical provinces must necessarily have varied with the season; and the competition for the same feeding-grounds, between the invading and the retreating forms, must have been long, fluctuating, and severe. The passage, therefore, from the Pliocene to the Pleistocene fauna must have been extremely gradual in each area; and the lines of definition between the two must be, to a great extent, arbitrary, instead of being sufficiently strongly marked to constitute a barrier between the Tertiary and Posttertiary groups of life of Lyell, or between the Tertiary and Quaternary of the French geologists. The principle of classification which I shall adopt is that offered by the gradual lowering of the temperature, which has left its marks in the advent of animals before unknown in Europe; and I shall divide the Pleistocene deposits into three groups:—

1st. That in which the Pleistocene immigrants had begun to disturb the Pliocene mammalia, but had not yet supplanted the more southern animals. No arctic mammalia had as yet arrived. To this belongs the Forest-bed of Norfolk and Suffolk, and the deposit at St.-Prest, near Chartres.

2nd. That in which the characteristic Pliocene Cervidæ had disappeared. The even-toed Ruminants are principally represented by the Stag, the Irish Elk, the Roe, Bison, and Urus. Elephas meridionalis and Rhinoceros etruscus had retreated to the south. To this group belong the Brick-earths of the lower valley of the Thames, the river-deposit at Clacton, the Cave of Baume, in the Jura, and a river-deposit in Auvergne.

3rd. The third division is that in which the true arctic mammalia were among the chief inhabitants of the region; and to it belong most of the ossiferous caves and river-deposits in Middle and Northern Europe.

These three do not correspond with the Preglacial, Glacial, and Postglacial divisions of the Pleistocene strata in Central and North Britain, since there is reason to believe that all the animals which occupied Britain after the maximum cold had passed away, had arrived here in their southern advance before that maximum cold had been reached, or, in other words, were both Pre- and Post-glacial.

I shall first of all examine how this classification applies to Great Britain.

The third or late division of the Pleistocene strata will be taken first. The evidence that it is far older than any of the Prehistoric