Page:Popular Science Monthly Volume 30.djvu/614

592 cudweed (Gnaphalium supinum), the bog bilberry (Vaccinium uliginosum), the Alpine bearberry (Arctostaphylos Alpina), and the Lapland phlox (Diapensia Lapponica). Some of these, and others like them, were old friends already familiar to me on European mountains; but some were fresh American acquaintances, whose faces I was glad indeed to see on these bald summits. Both types, however, were alike in their thoroughly northern and almost arctic aspect; they were the plants of Greenland, of Finland, of the North Cape in Norway, of frozen Rupert's Land, of equally frozen Siberia. Some of them were common to both hemispheres, some were peculiar to the New World, but all, indiscriminately, were members of that same old circumpolar flora which came into being at the extreme end of the Pliocene period, when the world was just beginning to cool down at its extremities for the long secular winter of the Glacial Epoch.

Traces of that gradual cooling down are by no means wanting in the geological deposits of either hemisphere. The Pliocene period, as a whole, both in Europe and America, was an age of warm and genial climates, of large and vigorous animal types, of rich, sub-tropical-looking vegetation. In the Red and Norwich crags of England, for example, we find the remains of mastodons and elephants, of hipparions and hyenas, of the hippopotamus and the rhinoceros, of the tapir and the horse. In the rich leaf-beds of the nearly contemporary Vienna basin, we meet with a correspondingly warm sub-tropical flora—a vegetation abounding in sequoias, liquidambars, and chestnuts, fragrant with cinnamon, laurel, and tamarind. In the Loup River beds of the upper Missouri region, again (Professor Marsh's "Niobrara group"), America possesses similar mammalian remains of tropical and almost Oriental character—a tiger larger than the Bengal beast, an elephant, a mastodon, several rhinoceroses, and the earlier sketchy prototypes of the camels and the horses. The period when such warm-weather creatures flourished in such northern latitudes must surely have been one of very genial climatic conditions. But, toward the close of the Pliocene age, mutterings and forewarnings of the great glaciation begin to show themselves, and to herald the advent of that vast ice-sheet which gradually swallowed up in its devouring bosom the better portion of either continent.

Already in the Norwich crag of England the evolution of such northern molluscan species as Scalaria Grœnlandica, Panopœa Norwegica, and Astarte borealis (whose very names attest their arctic habits in our own day), gave evidence of a slow but certain lowering of the world's temperature. Nature only produces these cold-weather types where the surrounding conditions have rendered the change absolutely necessary. In the somewhat later Chillesford beds, the great invasion of arctic kinds begins in earnest; about two thirds of the shells whose fossil remains form the fauna of the period still survive in high northern waters. Slowly, as the period of greatest