We are led by the previous comparison, and on further investigation, to the app.-went fact, that some pertioue of the earth', surface, however, above the sea-level, present Us with the essentially unaltered configura tion of other portions which are still below it. From the united geo graphical researches and inductions of Dr. Thomas Thomson, Dr. Joseph D. !looker (` Himalayan Journals,' voL ii,, p. 399), and Professor James D. Forbes, it has appeared that the structure of Norway very closely resembles, on a small scale, that of Central Asia, particularly of the Himalaya ; and that if it were so elevated that the bottom of the deep fiords which penetrate it became dry land, we should have a model of the Himalaya, with its deep valleys and high acute :summits ; or, conversely, if the latter country were depressed, so that those valleys became ocean-hollows, the land and mountain-peaks remaining above the sea-level would correspond, on the great scale, to the moun tain-peaks of Norway, and the submerged valleys to the fiords. Dr. Thomson (' Western Himalaya and Tibet,' p. 492) has extended this comparison to the structure of Scotland, well known to resemble that of Norway, and to be, in fact, a continuation of it. lie has also described (pp. 429-438) the remarkable elevated plain of Karakoram, south of the pass of that name, occupying an immense concavity in the great chain of the Kouen•lua, the northern face of the table-land of Tibet. It measures from six to eight miles in diameter, and hafl a mean altitude of not less than 17,000 feet, being surrounded by great depressions separating it from the mountains. it has the appearance of having formerly been the bed of a lake, but is itself crossed by several ranges of hills, and by a rivulet, and has a water-parting. This is evidently the configuration of an island in the ocean, agreeing, therefore, with the examples above described.
Conversely, a portion of the bed of the Atlantic, from the coast of Ireland westward, has a configuration resembling that of the land of Western Europe ; but still further west, the great Atlantic depression, very unlike a terrestrial valley, commences abruptly, the depth of the sea increasing, within a distance of twenty miles only, from 1320 to 9000 feet, forming a sort of marine cliff on a gigantic scale. For at least a thousand miles the whole surface is one vast depressed plateau, which, according to Professor Ansted (in his Geological Gossip,' a work deserving of a more appropriate title), is "totally unlike any equal extent of dry land, though more resembling that on the eastern side of the Andes, in South America, than any other known Lend." On the (North) American side of the Atlantic plateau " there is a second cliff, facing eastward, having a total rise of about 5000 feet, imme diately to the west of which the ground slopes gradually upwards at the rate of about forty feet in a mile, till it reaches the American continent." We thus arrive at the final confluence of the two branches of the subject. The elevations and depressions of certain parts of the globe,
of various magnitudes, closely resemble some of the continental masses and ocean-hollows; while the latter more generally have a distinct and peculiar character ; and the corrugations of the upper surface of those masses, ordinarily termed mountains and valleys, in their actual con dition, usually differ from both, as already represented. In a paper On the Lines of Deepest Water around the British Isles,' by the Rev. It. Everest, F.G.S., read before the Geological Society of London, on June 19th, 1861, and which will probably appear in the ' Quarterly Journal' of that Society, the student will find new materials for pur suing this particular subject.
Some geographical observers, because a river-basin (defined in the article Ittvens) is necessarily bonnded by high land, except where it declines to the sea, employ the terms " basin " and " valley " of a river indiscriminately and convertibly. Thus, Mr. Alfred It. Wallace begins the chapter of his Travels on the Amazon and Rio Negro,' headed " The Physical Geography and Geology of the Amazon Valley," with the statement that " the basin of the Amazon surpasses iu dimension that of any other river." Now, although, strictly speaking, the differ ence between river-valleys and -basins is comparative only, and merges in nature, yet this practice leads to erroneous conceptions of fact ; for though a wide valley, through which a river of moderate dimensions having many small tributaries takes its course, may be termed a basin (sometimes called a valley-basin), yet the main stream, as well as its tributaries, must each have its own separate valley, however shallow : such is the Thames, which is described as having a basin, while its valley is often not recognised, though some of its affluents flow through well-characterised valleys. (THAMES, in Gum Div.] On the other hand, the basin of a great river may, and in fact must, include many such wide valleys ; and a river-basin, more properly, Is the country which is made up of them, and the drainage of which at last finds its way into one stream, and through one principal outlet into the ocean. In this sense, the depression through which the upper course and affluents of the river flow may sometimes be regarded as the basin, while that giving passage to the lower course is properly termed its valley. In the case of the Nile, the depression through which the single stream runs for 1300 miles, from the unction of its L•st affluent, the Atbara, to the Mediterranean, that us, its lower course, is properly the valley of that river ; while its basin comprehends a vast extent of country in Africa, to the eastward, southward, and south west of that point, made up of many valleys of large dimensions, some giving passage to its great head-streams and tributaries, and almost all to considerable rivers, the affluents of the former respectively.