NOTES OX THE CONSTITUTION OF CERTAIN ROCKS OF 

 THE ARCH.-EAN AGE IN THE PROVINCE OF NATAL. 



(Plates 3 and 4.) 



By James A. H. Ak.mstkoxg, I'.G. S., F.C. S. 



In a previous paper* I dwelt upon the existence of a narrow 

 belt of granitic, gneissic and schistose rocks that traverse the 

 Colony of Natal from north to south and which belong to the 

 Archaean Age. In this paper I propose to deal with some of the 

 more important rocks that would figure prominently in a vertical 

 section cut through the narrow belt in Southern Natal, in the 

 neighbourhood of latitude 30° 18' south and lying between 

 the 30° and 31° lines of longitude, and covering a distance 

 of approximately 29 miles. Foremost among them occurs a 

 whitish to greyish coloured gneiss which was dealt with macros- 

 copically previously and which was stated to consist for the most 

 part of quartz, felspar and mica. It is sufficient here to note 

 that outcrops of this scries are most numerous towards what may 

 be termed the central part or zone of this belt. Microscopically 

 this gneissic rock is seen to consist of a crystalline aggregate of 

 quartz, felspar, mica and, last but not least, hornblende. The 

 structure is holocrystalline and truly gneissic. The constituents, 

 particularly the quartz which makes up the greater part of the 

 rock, appear to be drawn out longitudinally in the direction of 

 foliation and the hornblende with a quantity of felspar and the 

 other minor accessories with small particles of quartz appear fo 

 be confined to the interstices between the quartz or the quartz 

 and felspar lenticles. The quartz crystals contain numerous liquid 

 inclusions and the action of the quartz in polarised light shows 

 that it has suffered from the effects of straining. A like effect 

 may also be observed in many of the felspars. Many of the 

 elongated quartz crystals seem to extinguish in polarised light in 

 successive parallel bands which vary in direction but slightly 

 from the minor axis of each of the lenticles so affected. The 

 hornblende is a dark greenish black variety, and the mica group 

 seems to be represented by biotite, but this mineral is not present 

 to any extent. Microcline is the chief felspar present and shows 

 the characteristic cross-hatching. Another variety of triclinic 

 felspar occurs and it also belongs to the acid end of the felspar 

 series. The triclinic felspar shows twinning after the Albite Law 

 and at times after both the .'Mbite and Periclinc Laws. In some 

 instances I have noticed that the twin lamina*, although most 

 marked towards the end of a felspar crystal, which had a tendency 

 to assume an idiomorphic shape, almost become invisible towards 

 the central parts of the crystal, or they may show a tendency to 

 become spindle shaped. At times the twin laminae, though 

 parallel, are not straight, but appear curvilinear or wavy. The 

 hornblende is sometimes visible in irregular patches and at times 

 shows a fibrous structure. The felspar is often so decomposed 

 as to be beyond recognition. The hornblende, though often 

 affected by decomposition, is not so badly affected as the felspar. 



* Vol. 6, pp. 123 et. seq. 



