The Russians chanced to cross it in the 17th century at its narrowest and most N. part, and thus struck the Pacific on the foggy and frozen shores of the Sea of Okhotsk; but two centuries elapsed ere, after colonizing the depressions around Lake Baikal, they crossed over the plateau in a more genial zone and descended to the Pacific by the Amur. After that they spread rapidly S., up to the nearly uninhabited valley of the Usuri, to what is now the Gulf of Peter the Great. In the S.W. higher portions of the plateau formation the empire has only comparatively recently planted its foot on the Pamir, and it was only a few years earlier that it established itself firmly on the high-lands of Armenia. A broad belt of hilly tracts—in every respect alpine in character, and displaying the same variety of climate and organic life as alpine tracts usually do—skirts the plateau formation throughout The its entire length on the N. and N.W., forming an inter- alpine mediate region between the plateau and the plains. The belt. Caucasus, the Elburz, the Kopet-dagh and Paropamisus, the intricate and imperfectly known network of mountains W. of the Pamir, the Tian-shan and the Ala-tau mountain regions, and farther N.E. the Altai, the still unnamed complex of the Minusinsk Mountains, the intricate mountain-chains of Sayan, with those of the Olekma, Vitim and Aldan all arranged en echelon—the former from N.W. to S.E., and the others from S.W. to N.E.—all these belong to the same alpine belt that borders the plateau from "end to end of the series. The flat lands which extend from the base of the Alpine foothills to the shores of the Arctic Ocean, assume the character either of dry deserts, as in the Aral-Caspian depression, or of low The flat tablelands, as in central Russia and E. Siberia, of lands. lacustrine regions in N.W. Russia and Finland, or of marshy prairies in W. Siberia, and of tundras in the far N. Throughout the whole of this vast area, their monotonous surfaces are diversified by only a few, and, for the most part, low, hilly tracts. Recently emerged from the Post-Pliocene sea, or freed from their mantle of ice, they persistently maintain the self-same features over immense areas; and the few portions that rise above the general elevation have more the character of broad and gentle swellings than of mountain-chains. Of this class are the swampy plateaus of the Kola peninsula, sloping gently S. to the lacustrine region of Finland and N.W. Russia; the Valdai table-lands, where all the great rivers of Russia take their rise; the broad and gently sloping meridional belt of the Ural Mountains; and lastly the Taimyr, Tunguzka and Verkhoyansk ranges in Siberia, which, notwithstanding their sub-Arctic position, do not reach the snow-line. The picturesque Bureya Mountains above the Amur, the forest-clad Sikhota-ahn on the Pacific, and the volcanic chains of Kamchatka belong, however, to quite another orographical construction, being the border-ridges of the terraces by which the great plateau formation descends to the depths of the Pacific Ocean.