6.  In the United States, strata of Pliocene age are found in North and South Carolina.  They consist of sands and clays, with numerous fossils, chiefly Molluscs and Echinoderms.  From 40 to 60 per cent of the fossils belong to existing species.  On the Loup Fork of the river Platte, in the Upper Missouri region, are strata which are also believed to be referable to the Pliocene period, and probably to its upper division.  They are from 300 to 400 feet thick, and contain land-shells, with the bones of numerous Mammals, such as Camels, Rhinoceroses, Mastodons, Elephants, the Horse, Stag, &c.

As regards the life of the Pliocene period, there are only two classes of organisms to which our attention need be directed—­namely, the Shell-fish and the Mammals.  So far as the former are concerned, we have to note in the first place that the introduction of new species of animals upon the globe went on rapidly during this period.  In the Older Pliocene deposits, the number of shells of existing species is only from 40 to 60 per cent; but in the Newer Pliocene the proportion of living forms rises to as much as from 80 to 95 per cent.  Whilst the Molluscs thus become rapidly modernised, the Mammals still all belong to extinct species, though modern generic types gradually supersede the more antiquated forms of the Miocene.  In the second place, there is good evidence to show that the Pliocene period was one in which the climate of the northern hemisphere underwent a gradual refrigeration.  In the Miocene period, there is evidence to show that Europe possessed a climate very similar to that now enjoyed by the Southern United States, and certainly very much warmer than it is at present.  The presence of Palm-trees upon the land, and of numerous large Cowries, Cones, and other shells of warm regions in the sea, sufficiently proves this.  In the Older Pliocene deposits, on the other hand, northern forms predominate amongst the Shells, though some of the types of hotter regions still survive.  In the Newer Pliocene, again, the Molluscs are such as almost exclusively inhabit the seas of temperate or even cold regions; whilst if we regard deposits like the “Bridlington Crag” and “Chillesford beds” as truly referable to this period, we meet at the close of this period with shells such as nowadays are distinctively characteristic of high latitudes.  It might be thought that the occurrence of Quadrupeds such as the Elephant, Rhinoceros, and Hippopotamus, would militate against this generalisation, and would rather support the view that the climate of Europe and the United States must have been a hot one during the later portion of the Pliocene period.  We have, however, reason to believe that many of these extinct Mammals were more abundantly furnished with hair, and more adapted to withstand a cool temperature, than any of their living congeners.  We have also to recollect that many of these large herbivorous quadrupeds may have been, and indeed probably were, more or less migratory in their habits; and that whilst the winters of the later portion of the Pliocene period were cold, the summers might have been very hot.  This would allow of a northward migration of such terrestrial animals during the summer-time, when there would be an ample supply of food and a suitably high temperature, and a southward recession towards the approach of winter.