glaciers. Not only so, but by tracing the horizontal and vertical distribution of glacial phenomena we have been able to show what regions were wholly ice covered, to measure the thickness attained by ice sheets and glaciers, and to estimate the angle of their surface slope. It is, in short, quite possible now to draw maps of Europe which shall give a fairly accurate presentment of the aspect presented by our continent in glacial times. On maps of a sufficiently large scale we can delineate not only the great inland ice of the north and northwest, but the snow fields and numerous glaciers of the Alps and other mountainous tracts, together with the areas covered by fluvio-glacial deposits.
So much for what we may call the physical evidence. But this is not all, for associated with the true glacial accumulations occur in many places beds charged with the remains of arctic-alpine plants and animals. The evidence of fossil-organic remains, therefore, fully supports the conclusions arrived at from a study of purely glacial phenomena. We know that arctic forms of life lived in our seas at the time of which I am speaking, and that the countries outside of the glaciated areas were then clothed and peopled by an arctic-alpine flora and fauna.
But, as if in contradiction of this evidence, certain other deposits charged with the remains of temperate and southern species of plants and animals appear intercalated among the true glacial accumulations. The study of these and of their relation to subjacent and overlying morainic and fluvio-glacial accumulations has led to the conclusion that the Glacial period was not one continuous period of arctic conditions, but a cycle or succession of alternating cold and genial epochs.
So far as we at present know, glacial conditions first supervened in late Tertiary times—in the so-called Pliocene period. In the earlier part of that period the European climate had been singularly genial. Warm seas, tenanted by many southern species of mollusks, washed the shores of the British area, while the land was clothed with a much more varied and abundant flora than we now possess. Great forests seem to have covered vast areas, occupying not only the plains and the river valleys, but extending far up the mountain slopes of such regions as France without much change of character. The same species, indeed, appear to have flourished equally well in Cantal and central Italy. Some of these had come down from early Tertiary times and were destined soon to become extinct; some, again, were special forms belonging to genera which in our day are exotic; others were species which have survived to the present in more southern and eastern regions, while yet others are still represented in Europe by identical or very closely allied species. Thus the flora of the Pliocene was connected both with the past and the present plant life of Europe, while at the same time it had relations with the floras of distant southern and eastern regions—with Florida, the Canary Islands,
