PRESIDKXTIAL ADDRESS. SECTIOX B. 3I 



That some of the metamorphic rocks of Prieska and Ken- 

 hardt were volcanic rocks alHed to basalt is shewn by the pre- 

 servation of their amygdales and the mineral composition of 

 the bulk of the rock, even in cases where there is no re- 

 semblance to basalt in the minute structure. The usual rela- 

 tions of the felspar, augite and metallic ores in lavas have 

 been replaced by a granular structure in which no priority of 

 formation can be made out in favour of any one group of 

 minerals, though the changes in the nature of the minerals 

 themselves have probably been slight. Though the transi- 

 tional stages are known in certain groups of outcrops, they have 

 not been traced in one and the same lava-flow. Whether this 

 can be done remains for further work to prove. 



It is evident from the detailed examination of these rocks 

 that the transference of material from one part to another of 

 the same mass, or from one rock to another, has often been 

 extremely limited, but the proof of an extensive interchange, 

 or diffusion, could only be got by a much more detailed re- 

 search in the region than has yet been attempted. The indi- 

 cations of such a process are given by the massive or foliated 

 granitic rocks which cont;un large quantities of the magnesian 

 and aluminous silicates not found in the normal granite and 

 gneis,-. Such rocks are found in Kenhardt and Prieska, and 

 they can be looked upon as the result of the solution of sedi- 

 ments of a magma which would have solidified as granite or 

 gneiss had it not dissolved the sediments. 



The possible courses taken in the averaging of a large bulk 

 of sediments, lavas and the magma which carried with it the 

 means of bringing them to the liquid state, and the possible 

 re-differentiation of the resulting mixture, are beyond my 

 powers to follow; and it will be sufiicient to point out one cir- 

 cumstance which raises doubt as to the actuality of the pro- 

 cess. The inevitable result of the great changes at the sur- 

 face which bring about the formation of various kinds of sedi- 

 ments at the expense of igneous rocks is to separate the more 

 soluble from the less soluble constituents of the latter, and 

 therefore during past times the alkaline constituents have been 

 stored up in the ocean. The sediments available for a possible 

 melting up and re-issue as lava or intrusive rocks must have 

 suffered an impoverishment in alkalies, so we should expect, 

 on the supposition that such cycles of change are restricted 

 to a comparatively thin layer below the surface, that the later 

 igneous rock would contain distinctly less alkalies than the 

 earlier, yet no evidence of this has been discovered. 



One of the most important discoveries yet to be made in 

 South African geology is the position in the stratigraphical 

 succession of the equivalent of the Cambrian system of other 

 continents. Until Cambrian beds are recognised there can be 

 no thorough comparison of the ancient rocks with the Pre- 

 Cambrian of other countries. We only know that the ancient 

 rock-systems of the north, which may perhaps be of Pre- 

 Cambrian age, must represent an enormous lapse of time, and 



