THE CONSTITL HON OF CERTAIN ROCKS OF THE ARCH.^IAN AGE. 255 



Ihroughout these gneissic rocks are to be found bands of quartz 

 veins or leaders whose macroscopic features I referred to last 

 time. It will be sufficient here to note their lenticular shape, their 

 \-ariability in w idth and in dip and the general coincidence of their 

 line of strike with the lines of foliation of the component minerals 

 of the gneiss. Their dip always coincides with the apparent dip 

 of foliation in the gneiss. It is in these quartz veins I instanced 

 the occurrence of gold in Natal, and I may mention in passing, 

 from observations that I have made and which, though still incom- 

 plete, up to the present, show that it is not at all unlikely that 

 most of the gold that is to be found in some of the parts of Natal 

 rivers, the origin of which gold has long been a matter in dispute, 

 jnay have been derived from those portions of these quartz veins, 

 A\hich have now vanished through the denudation of past ages. 

 Livemember also that these quartz veins were found to be con- 

 .nected with pegmatites. Hand specimens often appear to be a 

 mass of pure quartz. Under the microscope this is found not to 

 be so. They consist of a crystalline aggregate of quartz crystals, 

 Avith some felspar and minor accessories, and possess a holo- 

 crystaliine structure and are of igneous origin. The quartz is 

 • clear and with numerous liquid inclusions. It is traversed in 

 ,parts by cracks and fissures. Within some of the quartz are 

 scattered minute laths of greenish black hornblende. Some of 

 the cracks must have been formed subsequently to the growth of 

 the hornblende crystal inclusions, for at times well-defined laths 

 .of hornblende are also cut by these same cracks. The liquid 

 inclusions often appear to be arranged in strings at right angles 

 to these cracks. The laths are irregularly dispersed throughout 

 the quartz but now and again a cluster may be found either with 

 their longer axes parallel to the strings of liquid inclusions or to 

 the fissures in the quartz. The quartz also shows that 

 it has been affected by strains, and as it is in crystal- 

 line continuity with the quartz of the gneiss such strain- 

 ing may have been coeval. There is a small quantity of felspar 

 of the variety microcline present and dispersed throughout 

 the mass in somewhat idiomorphic forms. In appearance and 

 optical characters it is the ^ame as that in the gneiss. \^ery 

 -often it is in a very high state of decomposition, producing the 

 same decomposition products as the felspar in the gneiss. A 

 ■ crystal of triclinic felspar of the van'ety albite may occasionally 

 be' detected in the quartz of the veins. Thus, then, we are led to 

 the conclusion that what was suggested at the previous meeting 

 from macroscopical features is borne out by microscopic analysis. 

 viz., that these quartz veins, which appear to be confined to this 

 type of gneiss and which cross the country for miles, represent 

 "igneous intrusions in the gneissic rocks by a somewhat more acid 

 magma at the time of or soon after the origin of the original 

 •gneissic rocks and before their conversion to a true gneiss. 

 Underlying this gneiss is granite varving in colour from white to 

 red — the variation in colour depending upon the colour of the 

 felspar present. The rock presents the crystalline structure visible 

 ^n ordinarv granites, and consists of an admixture of quartz, 

 'felspar, hornblende and a little mica. The quartz is similar in 



