30 PRESIDENTIAL ADDRESS. — SECTION B. 



Structure is not like that of ordinary sediments, lavas and in- 

 trusive rocks, and they are very old. Such rocks are found 

 over a large part of South Africa, and though they have re- 

 ceived some attention, they have not been so closely investi- 

 gated as analogous rocks in Europe and North America. 



There is no body of rock in this country which has been 

 shown to be older than the oldest recognisable sediments. The 

 great areas of gneiss and granite in the north and north-west 

 of Cape Colony, Rhodesia, the Transvaal and Natal, contain 

 streaks and blocks of other material, which, from its mineral 

 ■composition and the structures met with in it, was once either 

 sediment derived from rocks older than itself, and consequently 

 much older than the igneous rocks which envelope it, or vol- 

 canic rock poured out on the surface of the earth. In certain 

 cases there are transitions, in one group of outcrops, from 

 rocks that are clearly of volcanic origin to others in which the 

 original volcanic nature is only recognisable owing to the 

 preservation of lumps of quartz and other minerals that repre- 

 sent the amygdales of the formerly vesicular lava. The present 

 minute structure of these rocks is absolutely different from that 

 of any unaltered lavas, but is like that of schists and granulites 

 frequently met with in Archaean areas in many parts of the 

 world. Usually the different stages through which the rocks 

 have passed in reaching their present condition cannot be seen 

 in one and the same group of outcrops, but from the observed 

 instances of transitional rocks the origin of some of the ex- 

 treme results of change can be understood. 



In order to understand the relationships of many of those 

 highly metamorphosed rocks as seen in the field it appears to 

 be necessary to assume that absorption of the rocks invaded 

 by the once liquid granite has taken place on a large scale ; 

 yet, with the exception of a few comparatively small bodies 

 of rock with the structure of granite but having a mineral and 

 chemical composition very different from that of a normal granite 

 or other igneous rock, it is exceedingly difficult to point to the 

 only convincing class of evidence, that is, the existence on an 

 appropriately large scale of rocks v/ith a composition that re- 

 moves them from the ordinary igneous rocks, while in struc- 

 ture they belong to the latter. 



Frpm the nature of the case direct proof of the complete cycle 

 from a volcanic rock, through the various stages of conversion 

 into schists and granulites, and the return to conditions which 

 may result in its reappearance as an igneous rock can never be 

 obtained. Yet from a study of the intensely changed volcanic 

 rocks of Kenhardt and Prieska it is clear that a slight effort 

 will unable one to complete the cycle in imagination. If, how- 

 ever, the additional stages due to the selective action of the 

 various processes of weathering and deposition at the earth's 

 surface be introduced into the cycle, the following up of the 

 process becomes much more difficult, even in imagination, 

 owing to the necessity of providing for the renewal of the 

 more easily eliminated substances, such as the alkalies, and the 

 ■distribution of the accumulated alumina. 



