Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/33

 thus, proeeediug from Dordrecht to Aliwal, we find the level of the river at the latter place upwards of 4200 feet above the sea. Thence to Bloemfontein there is a continuous rise from one step to another, until the plateau upon which Bloemfontein is situated attains an altitude of upwards of 5000 feet ; while the mountains forming the dividing ridge between the heads of the various tributaries that run into the Upper Nu Gariep, or Orange river, and those that empty themselves into the Indian Ocean rise to a height of upwards of 10,000 feet.

The same continues to be the case in proceeding still further northward, across the upper portions of the streams that flow into the Ky Gariep, or Vaal. The culminating point of that part of the Draakensberg which forms the boundary of the watershed of this river is 7200 feet ; while the northern side of this same river- valley is bounded by an elevated plateau, called Hooge Veldt (fig. 4, A A), said to be, in portions near Magalies-Berg, nearly 7000 feet above the sea.

These elevated mountain-ranges, and the more level plateaux that are connected with them, form a vast but irregular semicircle, with an outline of upwards of 500 miles ; and within these widely separated points all the drainage flows either into the Orange or the Vaal river : but the extent of the successive plateaux of increasing altitude is confined within an equally irregular line, running round the interior, of this semicircle.

Beyond this line a vast denudation must have taken place, which has been no doubt one of the principal causes of the very considerable lowering of the level along these river-basins towards the west. This will perhaps be best explained, in the case of the Vaal, by a reference to the diagram that I have drawn to show the relative heights of the country (fig. 4), Here it will be seen that the plateau of Bloemfontein, together with the mountains with which it is associated, is the outer limit of the central portion of the elevated tract of country I have described, and extends between the basins of the two rivers. From this point towards the north-west, instead of rising step by step towards the interior, as has been supposed, the very reverse is the ease. It gradually descends for about 100 miles, until at Klip Drift, on the banks of the Vaal, its level is 1750 feet below that of Bloemfontein, or some 3498 feet above the sea, an elevation of not 50 feet above that of Queenstown, which is 280 miles further to the south, and about forty miles south of the Stormberg.

The great distance between Klip Drift and Pniel and the sources of the Vaal River makes the difference of altitude almost imperceptible to the casual traveller, as the Likwa (or Upper Vaal), rising near the Ipoko, is 330 miles, and the head of the Klipstopel (or Kapok, that is, Snow River, as it is called by the Dutch) is 420 miles distant.

The very considerable difference in altitude between these different points is sufficient to show the immense amount of denudation that has taken place. The fall must, of course, be still greater at the