Page:Popular Science Monthly Volume 65.djvu/72

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THE time has not yet come when the geological history of Asia can be written in full. It appears, however, that, with the exception of a marginal zone in the south, which belongs to the Himalayan upheavals, the great plateaus of east Asia are built up of crystalline unstratified rocks, granites, granitites, syenites and diorites, as well as of gneisses, talc and mica-schists, clay-slates and limestones, which all belong to the Archean formation (Huronian, Laurentian, Silurian and partly Devonian). They thus can not have been submerged by the sea since the Devonian epoch. The higher terrace of the plateau of Pamir and the plateaus of the Selenga and the Vitim are formed only of Huronian and Laurentian azoic schists; even Silurian deposits—widely spread on the plains—are doubtful on the plateaus. Their upheaval must date accordingly from an earlier age, and they must have been a continent during the Devonian epoch, while parts at least of the lower terrace were under the sea at that period. During the Jurassic and Tertiary periods immense fresh-water basins covered the surface of those plateaus; they have left their traces in Jurassic coal-beds, and in Tertiary sands and conglomerates, these latter appearing in mighty layers on the borders and slopes of the plateaus. The chains of mountains which fringe the plateaus along their northwestern and southeastern borders are of the same ancient geological origin. They rose above the Carboniferous, Triassic, Chalk and Jurassic seas which covered what are now the lowlands and lower terraces of Asia; the upheaval of these chains has, however, continued throughout these epochs, so that in the outer chains of Asia we see Carboniferous and younger deposits, up to Tertiary, lifted up to great heights. The same is true of the border-ranges along the southwestern border of the great plateau of east Asia, namely, the Himalayas, which were lifted during the Tertiary age, while at their northern foot, on the surface of what is now the surface of the plateau, traces of Triassic deposits seem to have been found near Lhasa. Carboniferous deposits are met with in Turkestan, India and western Asia; while in eastern Asia the numerous coal-beds of Manchuria, China and the archipelagoes are all Jurassic. As to the age of the plateaus of western Asia, it remains unknown at the present time.

  1. From an article in the March number of The Geographical Journal.