C. B. Horwood 8f A. Wade— The Old Granites of Africa. 465 



different from that of any of the gneissose or schistose rocks yet found 

 in Prieska, and it is therefore impossible to consider them as local 

 modifications of any of the latter, as the biotite-granulites may be 

 with regard to the gneiss. The amount of lime and alumina in the 

 pyroxene-granulites must be greater than is usual in igneous rocks 

 containing the same varieties of plagioclase. The granulites give one 

 the impression of being intrusive, but the question of their origin is 

 quite unsettled." 



I have quoted Mr. Rogers' description of these rocks in full because 

 he has given such a clear and detailed account of them, and because 

 there can be no doubt that the granulites of the Prieska district are 

 geneticall)^ connected with the normal type of granite, and Mr. Rogers 

 is evidently inclined to admit a difference in age between them and 

 the normal granite. Mr. Rogers^ also mentions the occurrence of 

 peculiar types of well-foliated gneiss amongst the Southern Bushmanland 

 granites and gneisses at the base of the Langeberg in Calvinia, which 

 consist chiefly of quartz, plagioclase, enstatite, hornblende, and biotite, 

 and occiu" in bands enclosed in gneiss of more normal character. He 

 further states that garnetiferous granite and gneiss are abundant in 

 that area. 



A description of certain microcline gneisses of the Congo Free State, 

 which are petrographically remarkably similar to some of the gneisses 

 already mentioned, will be found further on. 



Revieav of Work done during the Last Decade. 



Some of the work already done has of necessity been mentioned 

 under the last heading, but that which has not yet been alluded to 

 I now propose to review. 



In 1897 - Mr. A. R. Sawyer drew attention to what he called the 

 Rooi Kopjes mass of granite, situated south of Heidelberg between the 

 upper and lower Rooi Kopjes. The granite is here overlain uncon- 

 formably by a small outlier of Karroo Beds, and does not outcrop 

 at the surface, but has been encountered in bore-holes. According 

 to Mr. Sawyer's description it occurs at a considerable depth below 

 the level of the Witwatersrand rocks which outcrop around it, and 

 dip away from it along the margin of the Karroo Beds, which here 

 form the South Rand Coal-field. He stated " that the gneissoid 

 granite, forming the base of this coal-field, is part of the basement 

 underlying the whole of South Africa, and is here, in the writer's 

 opinion, distinctly anterior to the banket formation ". 



Later,^ in 1903, he states, in referring to his previous description of 

 this occurrence, that it appeared to him at that time " that the gneissoid 

 granite formed part of a protrusion of the vast underlying deep-seated 

 granite foundation which I have seen in so many parts of South Africa, 

 from the coast in Cape Colony to beyond Salisbury in Mashonaland, 

 and which I believe underlies the whole of South Africa, and that it 



1 Loc. cit., p. 91. 



- " The South Eand Coal-field and its Connection mth the AVitwatersrand Banket 

 Formation," by A. R. Sawyer: Trans. Inst. M.E., 1897, vol. xiv, p. 312. 



» " Remarks on some Granite Masses or the Transvaal," by A. R. Sawyer : Trans. 

 Geol. Soc. S.A., 1903, vol. vi, pt. iii, pp. 47-9. 



DECADE v. — VOL. VI. — NO. X. 30 



