"The broad belt of desert which traverses the eastern hemisphere, in a general direction from west to east (or, speaking more exactly, of W.S.W. to N.E.E.) reaching from the Atlantic on the one hand nearly to the Yellow Sea on the other, is interrupted about its centre by a strip of rich vegetation, which at once breaks the continuity of the arid region, and serves also to mark the point where the desert changes its character from that of a plain at a low level to that of an elevated plateau or table-land. West of the favoured district, the Arabian and African wastes are seas of land seldom raised much above, often sinking below the level of the ocean; while east of the same, in Persia, Kerman, Seistan, Chinese Tartary, and Mongolia, the desert consists of a series of plateaux, having from 3,000 to nearly 10,000 feet of elevation. The green and fertile region which is thus interposed between the 'highland' and 'lowland' deserts,[3] participates, curiously enough, in both characters. Where the belt of sand is intersected by the valley of the Nile, no marked change of elevation occurs; and the continuous low desert is merely interrupted by a few miles of green and cultivable surface, the whole of which is just as smooth and as flat as the waste on either side of it. But it is otherwise at the more eastern interruption. Then the verdant and productive country divides itself into two tracts, running parallel to each other, of which the western presents features, not unlike those that characterize the Nile valley, but on a far larger scale; while the eastern is a lofty mountain region, consisting for the most part of five or six parallel ranges, and mounting in many places far above the level of perpetual snow.