424 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



the dryness of its climate. The region has a pronounced dry season 

 through the months when the sun is north, and a considerahle rainfall, 

 some thirty inches, when the sun is south; and much of the rain falls in 

 heavy showers. The climate can not therefore be properly described as 

 arid ; yet arboreal vegetation is wanting, and even the smaller plant forms 

 are rather sparsely scattered, leaving much bare ground between them in 

 the dry season of our visit. The soil is probably more fully plant covered 

 after the summer rains have begun ; nevertheless the aspect of the region 

 is distinctly that of a dry country. The processes of weathering seem to 

 be not so much those of deep penetrating decomposition of the chemical 

 kind as those of shallow disintegration of the mechanical kind. The 

 processes of transportation are inactive in the dry season, but they must 

 work very effectively in the wet season, especially during and immediately 

 after the occasional downpours of rain that seem to be characteristic of 

 the region. At such times the smooth slopes of the unchanneled swells 

 must be swept by sheetfloods and the stream channels must be filled to 

 overflowing. The sheetflood would be greatly impeded if the Yeld sup- 

 ported a forest with thick undergrowth. The sheetflood is, on the other 

 hand, greatly favored by the actual scantiness of vegetation, and it can 

 hardly be doubted that erosion at this stage of the cycle progresses much 

 more rapidly in this comparatively dry region than it would under a cli- 

 mate moist enough to support an abundant cover of vegetation. It must 

 be chiefly by the action of the efficient sheetflood that the broad and gentle 

 swells have been and are still worn down without the formation of rill 

 channels and gullies. Kesidents in the region told us that sweeping- 

 floods on the plains, far outside of the stream channels, were of not un- 

 common occurrence from year to year. 



RIVER VALLEYS AND RIVER CHANNELS 



The unchanneled swells of the Veld are so broad that well defined 

 stream channels are few and far between. This is partly a feature of old 

 age, partly of dry climate, as stated above. At the time of our visit such 

 channels as occur were nearly or quite dry, and we had a chance of seeing 

 the banks and beds, which in a moister climate are usually hidden under 

 water. The banks were largely built of fine alluvium, from 10 to 30 feet 

 in height, and rock in place was visible in them here and there. The beds 

 were mostly covered with silt, sand, and gravel, but sills of rock were not 

 rare. The railway usually crossed the larger channels where rock sills 

 occurred, in order to have good foundations for the abutments and piers 

 of its bridges. The channels were sometimes so deep that they had the 

 appearance of young valleys, new cut beneath I lie plain, as if by the re- 



