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Page:Popular Science Monthly Volume 65.djvu/73

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69
GEOLOGY AND GEO-BOTANY OF ASIA.

What arc now the lowlands of Asia must have been widely submerged by the seas of the Tertiary period, as also those of the Quaternary (Post-Pliocene) period. During this last period, the whole of the lowlands of northwest Siberia were under the sea, as far as the fiftieth degree of latitude, a broad gulf of the Arctic Ocean penetrating at the eastern foot of the Urals as far as the watershed which now separates the basin of the Obi from the Aral-Caspian Sea. At the same time there are no traces of that sea on the high plains of east Siberia, which were only intersected by several narrow elongated gulfs of the ocean. The moistness that thus ensued permitted glaciers (which are wanting now throughout the middle parts of east Siberia and Mongolia) freely to accumulate, so that the whole of the upper plateau and its border-ridges were under a mighty ice-cap. Immense glaciers, like those of the Alps and Jura, covered also the alpine regions. How far glaciation extended over the plateau of Tibet and in China still remains unsettled. In Turkestan and Siberia immense accumulations of loess fringe the alpine regions; while in China they cover immense tracts, and are the most fertile regions of Asia.

Many important changes in the distribution of land and water have been going on in Asia since the Glacial period, and even during historical times. Since the Aral-Caspian Sea became isolated from the ocean, its desiccation, as well as that of the numberless lakes which dotted the surface of Asia during the Lacustrine (Post-Glacial) period, has proceeded with a rapidity which may be guessed from the very rapid rate at which the process has been observed to go on in Siberia during the last hundred years. All Asia bears unmistakable traces of having been covered during the Lacustrine period with numberless large and small lakes, which have now disappeared, not in consequence of the action of man, but in consequence of some general causes affecting the earth's surface since the last Glacial period. The process is still more accelerated by the rapid upheaval of the continent—the whole of the Arctic coast, as also most of that washed by the Pacific, the Mediterranean, the Red Sea and parts of the Indian Ocean, being in a state of gradual elevation, while the few areas where traces of subsidence have been noticed are very limited. The influence of the desiccation of Asia has been felt even during historical times, and the migrations of the Ural-Altaians, Turks and Mongols will probably be best explained if this change in the condition of central Asia be taken into account; while the same circumstance explains the present nearly desert state of those regions which were the cradle of European civilization.

Volcanoes play an important part in Asia's geology; no less than 122 active volcanoes are already known in Asia, chiefly in the islands of southeast Asia, the Philippines, Japan, the Kuriles and Kamchatka,