When the mountain-region through which the upper course of a river lies descends with less rapidity, and consequently occupies a much greater extent of country, the mountain-streams, as well as their Lanka, present very different features. Beth the streams and the !ankle show that the descent of the whole mass is not by a regular slope, but is formed by an alternation of plains and declivities ; iuu ascending such a mountain-stream, it is found that in certain places the rocky masses approach so near to the banks, as to leave hardly room enough for the river, and in these narrows the current is extremely swift, and generally a continnal rapid, interrupted by falls of moderate height.. These narrows however rarely extend more than few miles. Above them the mountains recede to some distance from both sides of the river ; and thus a basin is formed, in the middle of which the river flows with a comparatively slow current., not over bare rocks, but over a gravelly bed, and between low banks of earth. The bottom of the basin is level, or descends with a gentle slope, and may be cultivated or used as pasture-ground. In some of the rivers which descend from the central and eastern Alps, this alternation of narrow plumes and basins occurs several times. Thus the Reuses along which the great road runs which loads over the mountain-pass of St. Gothard, rushes with incredible velocity through the ravine of the Ilospendal, and falls 1800 feet before it reaches the basin of the valley of Urseru, which is nearly eight miles long and more than half a mile wide, and in which it rune with a gentle courso. At the northern extremity of the valley of Urnern tbo river enters the second narrow at the Urnerloch. This narrow, which extends about three miles to Geshinen, is extremely contracted, and within these limits the river descends 1074 feet, forming a succession of small cataracts. Below this is the basin of the Krachenthal, which is not so wide as that of and ibout six miles long. The course of the river within this basin is rapid, but there are no cataracts. From this valley the river escapes by the third narrow, which is about four miles long, and also very :extracted; it terminates at the village of Am-Stiig, where the Reuss t.nters the valley of Uri, in which it flows until it mingles its waters with those of the lake of Uri (Urnerasee), as the southern part of the Vierwaldsthdter-see is named. The same conformation is observed in the southern declivity of the Alps, where the river Ticino descends from the mountain-pass of St. Gothard. This river runs in a ravine from the Hospendal to Airolo, in which it descends about 2880 feet. It then enters the upper valley of Leventina, which is about seven mice losig and half a mile wide, and in which the river is rapid, bat has no cataracts. It issues from this valley by a narrow about two mike leas. hoteven 11 Dazio and Feido, where a series of beautiful mita* LIM occur, and the ravine is so narrow that an artificial road has been cut on the adjacent mountain called the Platifer. At Fahlo the Tuttle enters the middle valley or Leventina, In which it flows with rapidity to Glornico, a distance of about fifteen miles, but without forming any falls. The valley is leas than half a mile wide, and often interne dal rocke. Above (Bernie., the river enters a Abort narrow, at the outlet from which it ferias cataracts, and then melte. the wide valley called the Lower Valley of l.eventina, in which it flows with a companttively gentle course to Lago Mags,iere. The greater lumber of the riven' which originate in the Alps and Pyrenees are of this latter description. The basins which occur in these river valley' mawat some remote period have been filled with water, and this may 'have been drained off by the rivers forming an outlet for the waters by the narrows which now connect their basins with one another.
.t rcmaikap he peculiarity of most of the rivers joining tho Nile, dependant of course on the structure of the country through which they flow. is that they have a circular, or rather a spiral course, so that sitter having described a curve of greater or lees extent, round the ise lased moluitain masses which break the uniformity of the table lAtbn of Al yarinia, they return upon themselves at a comparatively abort from their sources. As instances of this may ho
tnentioned the Mareb, or I:her-el-Gash, the Bellegas, the Abai, the I:like. the Uoljeti In some place* the elevated mountain-ridges border immediately on low plaina In such mete the rivers mina be said to have a middle course; for as soon as they reach the plain their character is changed, and the rapid torrent is converted into a gentle stream. Thus the Marabou, later issuing from the Pongo de Manseriche, and entering the gnat plain, flows slowly through the alluvial level ; and tho Ganges, after leaving the Himalaya Slountains, at Htinlwar, flows with great ben& thnstgla the immense plains of India. [HINDUSTAN, in 1,;ff.till. Inv.) All tho rivers which descend from the southern declivity of the Alps to the plain which the river Po traverses are of the same description. In most cases, however, the tnountain-resciona are not in immediate contact with the plains, but are separated from them by hilly tracts, and that portion of the course of a river which lies through such a hilly region is called the middle course. The rocky masses rarely approach the bed of s river which has a middle course, but retire to some distance frorr them, so as to form between the higher grounds a wide valley, whirl the inundations of the river have covered with a thick layer of alluvia roil. It is remarkable that tho highest ground of these valleys occurs without exception, on the very banks of the rivers, and that the Land slopes from them towards the base of the higher grounds Accordingly the inundations generally cover the lower tracts, wide' are at some dietariee from the river, to the depth of several feet, whit. the banks are still above the surface of the water. The slopes of tic higher grounds, which may lie considered as the outer banks of the river, because they fix a limit to its inundations, are generally gradual and covered with vegetation. Tho current of the river itself is gentle This change, when compared with that of the mountain-stream, partly due to the more gentle descent of the hilly region, and partly b the form of its course. The bed of the river rarely lies in a strtugh line, hot continually forms bends, which are not acute angles, as in th, use of the motudainsstrearna, but have only a small curvature, so tha the river runs through the valley in a serpentine course. This circum stance renders the course of the river much longer than it would be i it dewed in a straight line, and consequently diminishes the fall am the rapidity of the current.
It is observed that rivers form numerous small islands and sand banks a short distance below the place where they issue from mountain-region. Thus tho Rhine, between Basel and Kehl, Strasburg, and the Amazonas, below the Pongo de Manseriche as fa east as the mouth of the Vapura, and the Mississippi, between month of the St. l'eter river and that of the Missouri, form Island and vand.banks. This is easily to be accounted for, by observing tha the river, on issuing from the mountains, retains a large quantity o earthy matter in suspension, which subsidy when the current decrease in rapidity. This sediment forma inlet. and sand-banks. Though i rarely happens, as already observed, that the rocky masses approacl close to the tanks in the middle course of a river, yet this generall occurs several times, and at such places the river usually forms rapid and whirlpools. A ledge of rocks traverses the bed of the river write places. Such ledges occur in the Danube at Protean, nea seuburg above Vienna, suer Preeburg in Hungary above Pesth, and a Orahova, or Orsova, on the boundary-line between Austria and Turkey On the Rhine they occur only between Mainz and Bonn, where th river is traversed by three ledges, at Bingen, at St. Gear, and nea Andernach respectively. Such ledges are found in nearly all the larg rivers of Europe. The elevations by which they are produced ar sometimes connected with ranges of hills.