PRESIDENT S ADDRESS. O 



lands under glacial control furnish a large part of their silicates 

 in a fresh state to the sediments accumulating on and around 

 them. These sediments, however, often contain fragments of 

 rock showing characteristic scratches, and they may present other 

 features which enable their recognition; angular grains are 

 abundant, there may be little sorting of large and small fragments, 

 and the iron compounds are not wholly oxidised to the ferric state 

 which gives a red colour to sands of the desert as well as to 

 much of the soil of the tropics. 



EVIDE^X•E FROM TOPOGRAPHY. 



The shape of a land surface is the result of all the 

 agents of denudation which have affected it since it first became 

 subject to their influence. The initial shape of the surface can to 

 some extent be pictured in imagination with the help of a know- 

 ledge of the structure of the rocks forming the area in question ; 

 thus the initial shape of our Karroo region, as indicated by its 

 structure, was one of low relief, without marked ridges, and the 

 areas of rising ground on it were due to piles of lavas and tuffs; 

 while the country to the south, where the Cape ranges now are, 

 almost certainly had distinct east and west ridges from its earliest 

 days, though these ridges became more and more pronounced as 

 the harder rocks in them became exposed. Though the structure 

 of a region has an important influence on the shapes subsequently 

 assumed down to a very late stage in the evolution of the surface, 

 the details vary with the agents at work. The final stage is a 

 low slope from the watershed to the sea, and it is rarely attained 

 in fact. The nearly flat surface, called a peneplain, can be 

 imagined to extend over a continent, but the peneplains of our 

 acquaintance are not so large; they are extending upstream 

 towards the watersheds, but we find hills rising from and around 

 them. To become a peneplain by the work of rain, wind and 

 rivers is the ultimate fate of the land, and it would be a plain of 

 erosion; but it is doubtful whether such a peneplain would result 

 under arid conditions during any one climatic period of which a 

 geologist could admit the duration.* An arid peneplain, such as 

 the Kalahari and Bushmanland, is probably a composite thing, 

 made partly of flat-cut rock and partly of filled-in valleys. The 

 isolated hills rising from a peneplain in an arid climate usually 

 have a peculiar form; they are made of hard rocks. They have 

 straight slopes, and their base is very sharply defined by the 

 sudden change of slope where they rise from the plain. Hills 

 of this type have been called Inselberge, island-mountains, a good 

 descriptive term.^ Many of them have no rock fragments lying 

 more than a few yards from the lowest outcrops owing to the 

 cover of sand on the plain, and this sand buries the base of the 

 hill. Passarge* describes great plains in the Bechuanaland Protec- 

 torate cut nearly flat in hard rock, and he attributed the erosion 

 to wind alone. 



The depressions called pans or vleys^ in South Africa are 

 characteristic of dry countries, and their origin has been much 



