The Evolution of the Biver System of Griqualand West. 359 
sandstone plateaux. Below T'Kuip the Brak Eiver flows in a moderately 
broad depression excavated in tillite to a depth of 200 feet below the 
general surface; along this section it receives no important lateral 
streams, the area both to east and west being characterised by the 
depressions known as ''pans." In the Brak Eiver catchment area gravel 
terraces occur at various altitudes and sometimes at a distance from the 
present channels ; the boulders consist principally of dolerite, and 
lydianite and quartzite produced by the contact action of the intrusions 
upon the Karroo shales and sandstones. 
Origin of the Veld. 
Under this title Professor Davis * has discussed a number of possible 
theories as to the development of the present surface features of central 
South Africa. Passarge f has pointed out how a peneplain might gradually 
be formed in an arid region and how it might subsequently be dissected 
either by warping or else by a change to a more humid climate ; he has 
suggested that this might be the case with the Kalahari, and that the 
latter, and consequently the Karroo as well, might have been dissected 
at their present level. 
Against this view we have the improbability that the Karroo was 
elevated continuously several thousands of feet, for the nature and 
position of the cretaceous rocks and the recent marine terraces in the 
coastal region show that this elevation proceeded by steps and that 
the upward movement has continued down to within recent times. 
Peneplains are, moreover, normally developed at low levels with respect 
to the surface of the ocean. With regard to the question of climatic 
variation, Passarge | has elaborated a cycle of changes from humid to 
arid and back to humid conditions in Tertiary and Pleistocene times. 
It is not yet known, however, to what extent the phenomenon of the 
silicification of sands and limestones can be relied upon as indication of 
a former arid climate. 
The evidence in Bechuanaland,§ slight as it is, is in favour of some 
variation in climate, and it seems not unlikely that during a certain stage 
in the elevation of the continent more arid conditions prevailed over 
northern Gape Colony at least. 
It cannot be denied that, as stated by Davis, the Karroo exhibits a 
decidedly mature type of erosion ; this can be well seen in Hopetown and 
* W. Davis, I.e., p. 435. 
t Passarge, " Zeitschr. d. Deutsch. Geol. Gesellsch.," Ivi., 1904, protokol, p. 193; 
" Naturw. Wochenschr.," new series, vol. iii., 1904, p. 657. 
I Passarge, " Die Kalahari," Berlin, 1904. 
§ Ann. Rept. Geol. Commn. for 1907, pp. 155-7. 
