Page:Popular Science Monthly Volume 6.djvu/697

Rh to the time of the above-mentioned submergence of so much of Europe, the countries mentioned were so high above the sea-level of the time that the bottom not only of the Straits of Dover but of a large portion of the North and Baltic Seas was dry ground. These waters have a depth even now of only some 200 feet—rarely of 250 feet—so that an elevation geologically very slight would expose the bottom of these shallow seas. So Ireland was then connected with Great Britain, and the latter with France; While Africa and Europe were joined by bridges of land, so to speak, e. g., by way of Gibraltar, and again by way of Sicily and Malta. As a natural result and confirmation of the latter instance, we find on these islands and the contiguous main-lands the remains of mammals peculiarly African, especially certain species of elephants. These strips of dry land divided the Mediterranean into several inland seas. If man existed in that remote epoch, as many arguments tend to prove, he could cross dry-shod between Africa and Europe, as did the animals just mentioned, and was a witness of a distribution of land and water in Europe and adjacent lands widely different from that of our day. There was also going on about him and beneath his feet the slow rising and sinking of vast continental and insular territories—processes requiring thousands of years for their accomplishment—and, if man's existence during them be admitted, constituting another proof of the great antiquity of the race.

It is to these slowly-effected but most important alterations in both the contour and relief of the surface that we must ascribe the great changes of the climate, not alone of separate localities, but of entire continents; and this conclusion we will finally use in explanation of the varied phenomena of the Ice period, with which we have specially to do. The Ice period, we have said; but, without attaching much importance to the astronomical influences previously mentioned, we are compelled to believe in a succession of Ice periods, the evidences of which are believed to be furnished in the several series of deposits that are assigned to corresponding epochs.

During the latter portion of the Diluvial period the earth acquired substantially the same relief as it has at present. The chief mountain-ranges, the Juras, the Vosges, the Black Forest, the Pyrenees, the Alps, etc., were then about what they are now, though somewhat higher relatively both to the sea-level and to the subjacent plains; for, by the operation of various natural forces, peak after peak has been either shattered and cast down, or slowly worn away, and their débris, carried down in the form of sand, gravel, or larger masses, have gradually but considerably raised the level of the valleys and plains.

We now proceed to consider the several subdivisions of the Diluvial and the Post-diluvial but prehistoric periods, and the traces of human existence belonging to each.

—The loëss, the layer of calcareous loam, sand, and gravel, with which our hills are covered, is full of the