When we are contrasting the vertebrate contents of two sets of superimposed strata of the Cretaceous, Oolitic, or any other ancient formation in which the shells are identical in species, we ought never to lose sight of the possibility of their having been separated by such intervals or by two or three thousand centuries. That number of years may sometimes be of small moment in reference to the rate of fluctuation of species in the lower animals, but very important when the succession of forms in the highest classes of vertebrata is concerned.
If we reflect on the long series of events of the Pleistocene and Recent periods contemplated in this chapter, it will be remarked that the time assigned to the first appearance of Man, so far as our geological inquiries have yet gone, is extremely modern in relation to the age of the existing fauna and flora, or even to the time when most of the living species of animals and plants attained their actual geographical distribution. At the same time it will also be seen, that if the advent of Man in Europe occurred before the close of the second continental period, and antecedently to the separation of Ireland from England and of England from the Continent, the event would be sufficiently remote to cause the historical period to appear quite insignificant in duration, when compared to the antiquity of the human race.
CHAPTER 15. — EXTINCT GLACIERS OF THE ALPS AND THEIR CHRONOLOGICAL
RELATION TO THE HUMAN PERIOD.
Extinct Glaciers of Switzerland.
Alpine Erratic Blocks on the Jura.
Not transported by floating Ice.
Extinct Glaciers of the Italian Side of the Alps.
Theory of the Origin of Lake-Basins by the erosive Action of
Glaciers considered.
Successive phases in the Development of Glacial Action in the Alps.
Probable Relation of these to the earliest known Date of Man.
Correspondence of the same with successive Changes in the Glacial
Condition of the Scandinavian and British Mountains.
Cold Period in Sicily and Syria.
EXTINCT GLACIERS OF SWITZERLAND.
We have seen in the preceding chapters that the mountains of Scandinavia, Scotland, and North Wales have served, during the glacial period, as so many independent centres for the dispersion of erratic blocks, just as at present the ice-covered continent of North Greenland is sending down ice in all directions to the coast, and filling Baffin's Bay with floating bergs, many of them laden with fragments of rocks.
Another great European centre of ice-action during the Pleistocene period was the Alps of Switzerland, and I shall now proceed to consider the chronological relations of the extinct Alpine glaciers to those of more northern countries previously treated of. [Note 32]
The Alps lie far south of the limits of the northern drift described in the foregoing pages, being situated between the 44th and 47th degrees of north latitude. On the flanks of these mountains, and on the sub-Alpine ranges of hills or plains adjoining them, those appearances which have been so often alluded to, as distinguishing or accompanying the drift, between the 50th and 70th parallels of north latitude, suddenly reappear and assume, in a southern region, a truly arctic development. Where the Alps are highest, the largest erratic blocks have been sent forth; as, for example, from the regions of Mont Blanc and Monte Rosa, into the adjoining parts of Switzerland and Italy; while in districts where the great chain sinks in altitude, as in Carinthia, Carniola, and elsewhere, no such rocky fragments, or a few only and of smaller bulk, have been detached and transported to a distance.