32 PRESIDENTIAL ADDRESS. SECTION B. 



that here no evidence has been found of a time when sedi- 

 ments first began to form on the earth's surface. The most 

 ancient rocks here are sediments of several kinds, including 

 limestones, and volcanic flows, which, allowing for the altera- 

 tion they have undergone, are of the same nature as rocks 

 formed in modern times. Then, as now, the predominant 

 source of sand and clay was granite; the debris was not always 

 broken up into sand and clay, for thick beds of quartz-felspar 

 grit were formed. The lime and magnesia furnished by the 

 weathering of rocks were to a large extent separated from 

 the siliceous and aluminous parts of the rocks and laid down as 

 beds of limestone, whether by organisms or not can only be 

 judged by analogy with younger limestones. The action of 

 air and moisture, wind and waves, may have been of about the 

 same order in those days as now, for all that we can determine 

 from the records preserved in the rocks. 



It must be granted that the precise significance of ancient 

 sedimentary rocks, which do not contain fossils, as to the 

 climatic conditions under which they were formed, is exceed- 

 ingly difficult to understand, but in the case of volcanic rocks 

 comparison with their modern analogues can be made on more 

 favoiu'able terms. 



There are at present no active volcanoes in South Africa, 

 but in the past this sub-continent has had its share of them, as 

 is the case with many other regions where there are none now. 

 But although the location of volcanic activity has changed, a 

 brief survey of the ancient volcanic rocks of the country and 

 a comparison with those of other countries will enable us to 

 form an opinion as to whether there is any reason to believe 

 that there has been a regular decline in vulcanicity during the 

 time represented by our rocks. 



Some of the earliest South African lavas yet recognised are 

 those of the Kheis series in Prieska, Kenhardt, and Gordonia. 

 There are probably two groups of them, one at the bottom 

 and one at the top of that formation. They are greatly altered 

 in texture from their original condition, but both basic and 

 acid lavas have been found in them, as well as beds of breccia 

 and tuff thrown out from the volcanoes, or derived from the 

 subaerial destruction of lava. The true thickness of these rocks 

 has not been estimated, but they may be no more than some 

 5,000 feet. The area they originally covered must have been 

 considerable, not less than 10,000 square miles, but at present 

 there is no information beyond this limit. Up to the present 

 time these and similar, possibly equivalent, lavas of the 

 Kraaipan beds, Bechuanaland, are the only volcanic rocks 

 older than the granite of Kenhardt, and also older than the 

 Ventersdorp ."iystem, that have been described from Cape 

 Colony, though the thick sediments of the Malmesbury series 

 are known superficially over a wide area in the south. 



The next great period of activity is represented by the lavas 

 of the Ventersdorp system, which, however, were very widely 

 separated in time from those just mentioned; the irruption of 



