One of the most striking characteristics of Asian river-systems is found in its double rivers, or two streams rising in the same region, flowing in almost parallel directions, and either uniting, or nearly so, before entering the sea. Among these twin rivers may be mentioned—the Syr-Daria and Amu-Daria, flowing into lake Aral; the Euphrates and Tigris, in western A., surrounding the plain of IlIesopotamia, uniting at Koona, and together flowing into the Persian gulf; the Ganges and Brahmaputra; and the Yang-tze kiang and Hoang-ho, in China, rising near each other, then widely separated in their courses, but again approaching each other, and both falling into the Yellow sea, only 100 miles apart.
The six great river-systems of A., rivers which will be found fully noticed under their respective names, are—the .Mesopotamian, that of north-west India. that of north-east India and Tibet, the Indo-Chinese, the Chinese, and the Siberian. The first comprises the two famous streams, the Tigris and Euphrates. The second comprises the Indus with its tributaries. The third system comprises the Brahmaputra and Ganges. The fourth system comprises the rivers of the Indo-Chinese peninsula; the chief of which are the Irrawaddy, the Slartaban or Saluen, the Mc-nam, and the Me-king or Cambodia. The fifth system is the Chinese. It comprises four great streams, all of which flow in an eastern or north-eastern direction into the Pacific; the Hong-kianis, or Canton river; the Yang-tze-kiang (or Son of the Sea); the Hoang-ho, or Yellow river; and the Amur. The sixth system comprises the large rivers of Siberia, the principal of which are the Obi, the Yenisei, and the Lena. They all have their sources in the Altaian mountains; flow n. or nearly so; and for 800 or 900 in. before their embouchure, traverse a dreary, flat, monotonous waste, until their sluggish waters creep into the Frozen sea.
geographical structure of A. is so complex, the different formations are so broken up and scattered. that a general description would be unintelligible. We must refer to separate notices, where'the geological structure and phenomena of circum scribed districts will be given in detail, and the reader will, in this way, be enabled to form a correct impression of the geology of Asia as a whole. See INDIA, CHINA, TIBET, etc.
iVatural vast extent of A., and its great diversities of climate, natu rally lead its to expect in it a great variety of natural productions, both animal and vege table. This expectation is heightened when we consider how completely this vast con tinent isdivided into separate portions by mountain ranges of great altitude, and how extensive the mountainous tracts themselves are. as well as the great extent of the ele
vated plateaux or table-lands, and when we add to these considerations that of the pecu liar character of wide regions—wastes of sand—level steppes—and extensive districts of which soil is strongly impregnated with salt. Accordingly, we find, both in the flora and fauna of Asia, all the variety which such considerations might lead us to expect.
The most northerly part of the continent, however, differs comparatively little in its productions from the corresponding parts of Europe and America. It exhibits the same arctic flora, with differences comparatively inconsiderable. Pines, birches, and willows form, as in the other continents, the last forests of the n. ; but upon account of the more severe climate, they do not reach a limit so northerly as in Europe. and particularly in the w. of Europe. Some of the common plants. of Europe are abundant as far e. as Kamtehatka: the crowberry (empetrnm nigrum), so plentiful in the moors of Scotland, is still more plentiful throUgh.out Siberia; the sauce vaccifikOilberries. etc.) and alibi (brambles, etc.) abound in the Kamtchatkan forests as in those of Scandinavia. There are, however, interesting differences. Heaths arc comparatively rare in Asia, its flora agreeing in this respect with that of America, rather than with that of The larch, which in Europe occurs only on the central mountains, extends far northward at the month of the Obi to the utmost limits of arburescent vegetation; probably it mere variety of the same species, although it has been described as distinct. In Kamtehatka, a different kind of birch replaces the common birch of Europe as a forest tree, and the Siberian stone pine is different from that of the s. of Europe. Siberia in its less frigid regions produces a luxuriant vegetation, of which herbaceous plants of unusually large size for a cold or temperate climate are a characteristic feature; as species of rhu barb, angelica, and cow-parsnip (heradeu at), some of which are now well known in Britain. It is indeed from the central and eastern temperate parts of Asia that the cultivated species of rhubarb are derived, and from the same region the rhubarb root, so valuable in medicine, is brought. In the abundance of grossulariacea3 (currants), the warmer parts of Siberia resemble North America, although most of the species are different.