The Evolution of the Biver System of Griqualand West. 357 
Rand limestones ; below the point down to Prieska its winding channel 
is hemmed in by banks of Dwyka tillite. All along the course of the 
river, and sometimes nearly 15 miles away to the south are terraces 
capped with gravels cemented with carbonate of lime, and usually pro- 
minent features in the landscape. For this reason the gravels, which 
appear to be diamondiferous, are unworkable. At Prieska the altitude of 
the river-bed is only 3,020 feet, so that the Kaap peneplain has been 
dissected to a depth of about 1,000 feet. The Riet Eiver is in 
many respects a duplicate of the Orange. Along with its tributary, the 
Modder, it drains a wide area of but little relief in the Orange Eiver 
Colony, enters a deep and narrow gorge cut in diabase below Modder 
Eiver Station, and for some distance before joining the Vaal Eiver has its 
channel incised in a terrace cut equally and uniformly in both shale and 
dolerite. 
A most interesting feature about the Yaal Eiver is that in a distance 
of 100 miles above Christiana the fall is only 30 feet, or less than 4 inches 
per mile, whereas from Warrenton to Barkly West the fall in 60 miles is 
just over 300 feet. 
In sawing downwards the Vaal Eiver has come across several of the 
buried ridges of the pre-Karroo diabase, and in them it has in time cut 
deep and steep-sided gorges. The two most important barriers are the 
one between Warrenton and Windsorton and that between Barkly West 
and Longlands. Above such a barrier downward erosion has been checked, 
while below it the Dwyka tillite has been rapidly removed ; hence at the 
lower end of each gorge are several gravel terraces occasionally with 
slopes connecting those at different levels, whereas above the barrier 
there is usually a single terrace the period of the formation of which is 
equal to the sum of those below the gorge. For this reason it is very 
difficult to correlate according to age terraces along different sections of 
the river. 
Along this portion of its course the Vaal Eiver forms a most interesting 
study, and shows in a clear manner the mutual dependence of different 
sections, the results of interference caused by minor obstructions, and the 
delicate readjustments of the rate and extent of the erosion in both a 
vertical and lateral direction. The peculiar hummocky surface of the 
diabase, with its Dwyka-filled, steep-sided, and ramifying depressions has 
obviously exercised an important influence in shifting the course of the 
river and its lateral streams from time to time. 
Gravels, usually diamondiferous, are found along the Vaal Eiver at 
altitudes up to 400 feet above its present bed preserved on slopes or 
shelves cut in amygdaloidal diabase, sometimes miles away from the 
river. 
Whereas the Vaal Eiver was hampered by hard diabase, in which it 
