356 
Transactions of the Boyal Society of South Africa. 
Dissection of the Kaap Peneplain. 
Through a renewal of river activity brought about by this elevation 
of the continent, aided by such tilting as may have occurred, the pene- 
plain was gradually dissected into its present condition. That the erosion 
was everywhere uniform in degree is shown by the relics of gravel 
terraces preserved here and there at different altitudes. The causes for 
these are, without doubt, complex, but the most important factors certainly 
are — intermittent elevation, climatic variation, and the existence of rock 
barriers athwart the stream-courses. 
The evidence derived from the coastal area, as shown by shelves or 
ledges at different altitudes,'''" points to. spasmodic elevation, but in the 
interior, which was presumably an area of lesser instability, this process 
was very possibly more uniform. The succession of gravel terraces is not 
necessarily a proof of intermittent elevation, for in a number of cases 
these shelves can in the clearest manner be seen to have been caused by 
the action of rock barriers, as these were one after the other exposed by 
the rivers in sawing down through the cover of Karroo deposits. The 
possibility of climatic variation will be considered later on. 
The elevation of the continent was no doubt due to isostatic adjust- 
ment following the denudation of its surface. 
The Orange Eiver from the Basutoland border down to Hopetown, 
flows at a depth of from 100 to 200 feet below the surface of the adjoining 
country. Wherever a dolerite dyke or sill crosses the river its channel 
becomes narrow and the gradient steeper ; above such a point there is 
usually a smooth reach for several miles. A good example of such a 
barrier is just above the railway bridge at Hopetown. Below this point 
there begins a peneplain at an average altitude of 3,600 feet above sea- 
level, and traceable over a considerable area in the Hopetown Division. 
Its origin may possibly be connected with the cutting of the channel of 
the Orange through the hard rocks below Prieska. At Hopetown itself 
this terrace is from 140 to 200 feet above the level of the present river, 
and the gravels contain abundant pebbles of amygdaloidal basalt from 
the Drakensberg, such pebbles being absent from the high-level deposits 
of the Harts and Vaal Elvers. Below Hopetown the Orange Eiver 
leaves the Karroo formation and plunges into a deep gorge cut in diabase 
and quartzite, and its rapid fall only terminates when it enters once more 
an area of Dwyka tillite and shale. From this point down to its con- 
fluence with the Yaal it courses along the base of a ridge of diabase rising 
in places to a height of 800 feet above it. At Eead's Drift it has sawn a 
narrow channel, with precipitous sides in a peneplain cut in the Campbell 
* E. H. L. Schwarz, Q.J. G. S., p. 70, 1906; Amer. Journ. Science, Sept., 1907, 
xxiv., p. 185. 
