The plains of India which skirt the foot of the southern face of the Tibetan table-land, for an extent of 1500 miles, nowhere have an ele vation exceeding 1200 feet above the sea, the average being much less, "The greater part of tho country between" the Sikkim Himalaya, forming part of that face, and the sea, as we are informed by Dr. Joseph D. Hooker, " Is a dead level, occupied by the delta of the Ganges and 13rahnutputra, above which the slope is so gradual to the base of the mountains that the surface of the plain from which the Himalayas immediately rise is only 300 feet above the sea." We have every reason to suppose that the plateau of Yerkend and Khotan, on the northern border of the table-land, like the country around Bukhara or Bucharia, lies at a very small elevatiou, probably not more than 1000 or 2000 feet above the sea, while the surface, as we know, and in agreement with a previous statement iu this article, descends on the borders of the Caspian to 80 feet below that level.
The table-land of Tibet itself is the summit of a great protuberance above the general level of the earth's surface, of which the Kouenlun and Himalaya are the north and south faces, while the other mountain ranges and intervening valleys commonly marked on our maps of Tibet are but corrugations of the table-land more or less strongly marked. In its general relief, however, the table land is laid out hori zontally. at a mean elevation of 15,000 feet. The Indus and Brabma pootra rivers maintain a course along the length of the summit of the table-land, and receive as they proceed the drainage of its entire breadth, with the exception, first, of an occasional strip along its southern edge, from which the water passes oft' more or less directly to the north through the Himalaya; and secondly, of some parts chiefly found in the northern half of the table-land, from which the water has no escape, but is collected in lakes in depressions on its very summit. The waters accumulated in these two streams are at length discharged by two openings in the Himalayan slope, through the plains of Hindos tan, into the ludian Ocean. None of the drainage of the table-land, so far as is known, passes in the opposite direction through the northern slope. The waters of that elope itself exclusively flow down to the plains of Yarkend.
That portion of the table-land which forms a plain along the upper course of the river Sutlej, we are informed by Major R. Strachey (to whose researches, and those of his brother Captain H. Strachey, much of our present accurate knowledge of these regions is due)," lies imme diately to the north of the British provinces of Kumaon and Gurhwal, and is about 120 miles in length, its breadth varying from 15 to 60 miles. Its surface, to the eye a perfect flat, varies in elevation from 16,000 feet along its outer edges, on the south-west and north-east, to about 15,000 feet in its more central parts, where it is cut through by the river Sutlej, which flows at the bottom of a stupendous ravine, formed out of the alluvial matter of which the plain is composed to a depth of 2000 or 3000 feet, and at its west end even more." But the table-land itself, it must be remembered, Is a mountain-mass. " The so-called plains" of Tibet, Dr. Hooker remarks, "are the flat floors of the valleys, and the terraces on the margins of the rivers, which all flow between stupendous mountains," either those of the northern and southern elopes or faces, or of the corrugations already mentioned.
In South America, contrasting with the lofty plains of Quito, of Santa Fade Bogota, Ste., are the Hanes and the plains of the Amazon ; while in North America, the interminable prairies and the low swamps round New Orleans form a striking contrast with the Rocky Mountaina and the elevated plains of Mexico.
Of Africa comparatively little is known ; but if the plains of Lower Egypt and part of the Sahara are very low, there are high plains in some of the mountainous regions.
The great plateau or table-land of eastern Africa, according to Dr. Beke, to whose continuation southward of Dr. Itiippell's Investigations we mainly owe our present knowledge of it, begins to the south of the country of Take, in about 15' of north latitude, where the anticlinal axis between the Nile and the Red Sea rises rapidly till it attains an elevation of 7000 or 8000 feet above the level of the ocean. At Halai, at the summit of Mount Taranta, not more thau eighteen geographical miles from Zulla (the ancient Adule, recently, 1860, taken possession of by the French government), near Massowah, the edge of the table-land his an absolute elevation of 8625 feet, which gives a rise of 1 in 12.7,
equal to an angle of 4° 30' with the horizon, to the eastern elope of the table-land (or, as it may be more -correctly called, in Dr. Beke's opinion, broad mountain-chain of Abyssinia). The western counter-elope towards the interior of the continent has a fall of 1 in 3431 only, giving an inclination of 10' ; consequently, on a direct line from east to west along the fifteenth parallel or north-latitude, the eastern elope of the Abyssinian mountain-chain (or table-land) towards the sea is, to the western counter-slope towards the Nile, as 23 to 1. But on a line corresponding with the courses of the principal rivers from south-east to north-west, the eastern elope has a rise of 1 iu equal to an angle of 1* 41', and the connter-slope of 1 in 460, or 7' 30", giving the proportion of 11.8 to 1. In making this estimate, however, the rise of the eastern slope is not taken from the level of the sea, but from that of the river Ilawash, which is the recipient of the waters of the eastern elope as the Nile is of the western, and has itself an absolute elevation of 2200 feet, at a point distant from the sea about 200 miles. This, again, gives a fall of about IL in 550, equal to an angle of 6' 15", for the eastward dip of the comparatively low-level country between the })awash and the Indian ocean. " As regard@ the counter-slope of the Abyssinian (Abyssinian) chain," Dr. Beke says," it would seem that the fall of the land towards the Nile in the western portion of it is con siderably greater than it is in the eastern ; so that the surface of the table land, or broad summit of the mountain chain, approaches more nearly to a level than if the slope were the same throughout. It is certain, however, that the table-land nowhere forme an absolute level, and that the general dip westward commences from its extreme eastern limit." This as the same geographer has shown, may, in the most general way, be compared with those of the Indian peninsula and South America, but with this difference : the Western Ghauts in the former, and the Cordilleras of the Andes, present their principal acclivities towards the west, and thence slope gradually eastwards ' • whereas the African plateau rises abruptly on its eastern aide, and has its western counterelope towards the interior of the continent and the valley of the Nile. Another point of difference is, that while the rivers which rise near the western edge of the Ghauts and of the Andes take their courses eastwards over the counter-slopes, at right angles with the water-parting (commonly called the water-shed) [Waren-SHED] or nearly so, and discharge their waters into the oceau—the streams which have their sources at the water-parting of eastern Africa flow in a general north-westerly direction, and fall into the Nile, which skirts the lengthened western counter-slope. To these com parisons of Dr. Beke, it may be added that the general structure of the African plateau resembles that of Tibet, described above. While the principal direction of the latter, however, is from east to west, its slopes being on the north and south—the principal direction of the former, as we have seen, is from north to south, the slopes being on the east and west. But, mutatis matandis, accordingly, the resemblance between the great table-lands of Africa and Asia is closer, we think, than that of the former to the elevated country bounded by the Ghauts and the Cordilleras respectively. In this comparison the lnaus and the Brahmaputra correspond to tho Nile itself, while their main tributaries, the Juinna, Ganges proper, Sze., answer to the streams which fall into the Nile, as already mentioned, from the south-east. The structural resemblance between the table lands of the two great continents is also very near in another respect, allowing for the different distribution of dimensions. In the southern extension of the African plateau, the rivers flow through deep trans verse valleys, forming openings through the eastern slope into the low country at its base, and thence into the ocean ; just as the waters accumulated in the two great Indian rivers are discharged by openings in the Himalayan or southern slope of the Asiatic) table-land, through the plains of Hindostan, into the ocean.