12 president's address. 



that of the whole system.^" The result is that below the con- 

 fluence the Orange is a stranger in the desert, an allocthonous 

 river in Penck's phrase, and is mei*ely a channel conveying water 

 from the east to the Atlantic without receiving noteworthy 

 additions on the way. The erosion accomplished by the Orange 

 Eiver is done by this water from the east, and consequently 

 the channel is deepened at a rate with which the lower affluents 

 cannot keep pace, for they are periodical streams from drier 

 regions. These tributaries are kept in a condition favourable to 

 active downward cutting near their mouths, and are in con- 

 sequence steeply graded in the lower parts of their courses.^* 

 The largest tributary is the Hygap or Molopo, draining some 

 87,000 square miles, about three-quarters of that of the com- 

 bined catchments of the Vaal and Orange head streams, but the 

 drainage basin is, in general, an arid country and it includes the 

 southern Kalahari. When Moffat visited the mouth of the 

 Molopo (Aintas as he called it) in 1854, he was surprised to find 

 it only 80 or 90 feet wide.^^^ There is as yet no large scale map 

 of that part of the country, but the contoured Reconnaisance 

 sheet of Kakamas, published in 1914, gives some important facts. 

 The mouth of the Hygap is about 1,500 feet above sea level, the 

 average slope of the last six miles of its bed is I'OO feet a mile, 

 and from near Molopo Kop, 20 miles up the valley, it falls 900 

 feet. This is a very steep grade for the lowest stretch of a 

 river draining a large area, and it is clearly due to the great rate 

 at which erouon is performed by the storm water in it as com- 

 pared with the slight erosion higher up. About Zwart Modder 

 the bed rock is buried under sand for some hundreds of miles. 



The principal feeders of the Hygap are the Nossob, Molopo 

 and Kuruman Elvers, and of these the Molopo, in certain parts 

 of its course,^* is a trench deeply cut in quartzites, as is tho 

 Hygap at Zwart Modder. These trenches are being filled in 

 with sand and limestone, proving that less water runs in them 

 than at some former period. So far as I can ascertain there 

 is no tributary from the southern Kalahari between the Nossob 

 and Molopo; if there ever were such a river its valley seems to 

 have been obliterated. The Molopo itself rises in the Western 

 Transvaal, on the Dolomite, and the exposure of that forma- 

 tion in which rivers tend to disappear owing to the passage of 

 the water underground through solution channels, must have 

 affected adversely the supply to the river above Mafeking. The 

 whole of the Western Transvaal was formerly covered by com- 

 paratively impervious Karroo rocks, at a date geologically recent, 

 and their removal by denudation has influenced the surface 

 drainage by allowing much of it to pass underground and escape 

 elsewhere than down the main rivers of the area. The Kuruman 

 river has been influenced in the same way; its chief gathering 

 ground is the Dolomite of the Kaap Plateau, which has been 

 denuded of its former cover of Karroo beds. In each of these 

 instances the area of higher rainfall in the east has had its run- 

 off dirhinished through the change from an impervious surface 

 to a limestone surface which has a steadily increasing capacity 



