338 THE GEOLOGICAL FORMATION OF GERMAN S.W. AFRICA. 



popular name. On following them up to windward, namely south, 

 they generally increased in number until a large mass was en- 

 countered. In several instances I noticed the remains of Hotten- 

 tot habitations at these accumulations of shells. In one case a 

 little excavating revealed the remains of a fire-place and a piece of 

 clay pot. 



In this desert country, with hardly any vegetation, what is more 

 natural than that the original inhabitants should go to the sea 

 shore for their food supply ? 



Several burying places of Hottentots are seen near the coast. 

 The decisions finally arrived at are that the solidified gravel is 

 derived from the loose gravel and not vice versa ; that the 

 Diamond-bearing gravel is a recent deposit (that is, geologically 

 recent) and that it is not of marine origin. What then is its 

 origin? Let us examine the facts as we find them : Here we have 

 a land surface, which, in the absence of evidence of submergence, 

 we can assume to have been a land surface from the earliest 

 Archaean times to the present day — a lapse of time which we are 

 utterly unable to realise and which may practically be described 

 as an eternity. Somewhere near the Carboniferous or Permian 

 Periods — a few million years either way are of no account — an 

 enormous number of pipes of Diamond-bearing "blue ground" 

 were formed, the actual rock being brought up to the surface and 

 most probably considerably overflowing it. In the natural course 

 of events denudation proceeded to an enormous extent and the 

 decomposed " blue ground " was scattered far and wide. 



When we consider that a large number of diamond pipes have 

 been discovered more or less by accident, and that our prospect- 

 ing and mining operations affect an infinitesimal proportion of the 

 earth's surface, it would be absurd to assume that there are not 

 a vast number of diamond pipes still undiscovered. The debris 

 from these pipes, formed during the denudation of the surface to 

 its present level, will of course form a very small oroportion of 

 the debris formed from all the other rocks that have been degraded, 

 except where the conditions for concentration are favourable. 

 Concentration can be effected in the following ways : — 

 (i). By means of moving water. 



In this way deposits of diamond-bearing gravel have been 

 formed on the banks and in the beds of rivers, and many such are 

 at present being worked. Owing to the mechanical action of the 

 water being very irregular, as it depends on the rainfall, this 

 method of concentration is not ideal and these deposits are natur- 

 ally irregular in their occurrence and richness. 

 (2) By means of wind. 



It would be difficult to imagine more favourable conditions for 

 wind concentration than exist on the coast belt of German South- 

 West Africa. A very small rainfall, the prevalence of furious and 

 well nigh incessant trade winds, and the presence of a rock surface 

 which is rather resistant to the chemical action of air and moisture, 

 have resulted in the concentration of coarser particles of debris in 

 the depressions in the rock surface and the removal of the fine 

 material which has been blown towards the equator. 



