THE CONSTITUTION OF CERTAIN ROCKS OF THE ARCH.liAN AGE. 261 



ihe base, is the usual dark greenish black variety, and is present 

 in some quantity, and is further marked as occurring chiefly m 

 irregular patches. The minute fragments of quartz show signs 

 •of having borne some degree of strain, but twinning in the felspars 

 is rarely to be seen. As with the more porphyritic variety, so in 

 ihis may be noted accessory minerals, such as zircons and mag- 

 iietite. With these remarks, then, may be drawn within a certain 

 •degree of certainty certain conclusions as to these ancient rocks in 

 ihis belt ; — 



(i). The original rock seems to have been a greyish to reddish 

 coloured granite with hornblende conspicuously present as 

 one of its constituents. 

 (2). Overlying the granites are gneisses which must have been 

 derived from the original granites through metamorphic 

 agencies. 

 (3). Throughout these gneisses but confined to certain belts 

 are bands of parallel quartz veins, which are associated 

 Avith ancient pegmatites, and which mark the occurrence 

 of intrusions in the gneiss at or soon after its origin but 

 prior to its metamorphism by rocks, probably parts of the 

 • same igneous magma, of a more acid character. 

 .(4). Intrusions in the gneiss by rocks of a dioritic type which 

 are evidentlv connected with and portions of the dioritic 

 rocks which are found on the coastal side of the narrow 

 belt and which may represent more basic portions of the 

 original igneous magma from which the gneiss was 

 derived. 

 (5). Intrusions in the gneissic, granitic, schistose and dioritic 



rocks by rocks of a typical diabasic character. 

 (6). The occurrence of metamorphic porphyritic rocks in the 

 inland portions of the belt and which undoubtedly were 

 connected with the original fluid magma and represent 

 more basic portions of it. 

 I may mention that there appears to be a two-fold division of 

 granites in Natal — a younger series and an older series — but, 

 much as it has been my desire to touch upon the younger series 

 and its pegmatites, and the differences between the two series 

 and other wide problems, my unaided investigations and efforts 

 have had to submit to the restraint of that desperate enemy 

 " Time." 



EXPLANATION OF PLATES. 

 I and 2. Vein Quartz with cracks and inclusions and decayed felspar. 

 3 and 4. Gneiss, showing cracks and decayed felspar. 

 5 and 6. Granite, showing state of decay of felspar. 

 7 and 8. So-called black porphyrite : felspar phenocryst in centre* 

 9. So-called black porphyrite, metamorphosed. 



10 and II. Quartz diorite. 



Nos. I. 3, 5, 7, 9 and lO in partially subdued transmitted sight. 



Nos. 2, 4, 6, 8 and 1 1 in polarised light. All magnified 60 diams. 



