F. P. MeiineH — Basic Dykes and Rock Genesis. 1 1 



are much too imperfect. The granite is pi-esumably of Archaean age, 

 and the dykes may be as late as Jurassic, though they may be* older. 

 That there was, however, a very great interval of time between the 

 consolidation of the granite and the intrusion of the dykes is beyond 

 dispute. We are certainly not dealing with products of related 

 magmas or of the same general period. 



Antelope Road Intrusion. This small intrusion of dolerite is situated 

 about 100 yards east of the road, some 3 miles from the Matopo 

 Hotel. It is a rather lenticular mass with straggly offshoots, and is 

 fine-grained throughout, being markedly chilled at its junction with 

 the granite, which is well exposed in a stream bed and on its banks. 

 At the contact the granite has its felspars reddened and its quartz 

 assumes a particularly glassy aspect, while the dolerite encloses 

 numerous xenoliths of the granite a foot or so in diameter. In spite 

 ■of chilling it has succeeded in insinuating itself among the constituents 

 of the xenoliths to a most remarkable extent, so much so that in 

 extreme cases almost every individual quartz and felspar grain is 

 isolated from its neighbours by dark strings of basic material. 



Microscopic Features. The dolerite is a granular medium- to fine- 

 grained type without any features calling for special remark. One 

 slide shows its junction with a granite xenolith [1737]. PL I, Fig. 1 

 .gives a good idea of its appearance, and it need only be noted that 

 much of the augite is uralitized. The edge of the granite xenolith is 

 illustrated in Fig. 2. As will be seen, the fine-grained dolerite forms 

 a kind of ground-mass in which the quartz and felspar of the granite 

 •are embedded like phenocrysts : the corrosion of the quartz and the 

 incipient disintegration of the felspar are well shown. [1738] is cut 

 from the central part of the same xenolith, several inches from the 

 edge. Its interpenetration by the basic material is as conspicuous as 

 near the margin. The matrix is of very fine-grained dolerite, with its 

 ferro-magnesian constituent represented by pale-greenish hornblende 

 and the iron-ore converted into leucoxene. Through this are distributed 

 numerous xenocrysts of much corroded quartz, usually surrounded by 

 a rim of fine-grained material, and others of felspar. The last are 

 very cloudy in appearance, so much so as to be almost opaque, but 

 the twin striations are not entirely obscured. 



[1735.] This represents the main mass of granite within a few 

 inches of the contact. It shows reddish altered-looking felspars, very 

 glassy quartz, and strings of dark greyish-green material, which at 

 •once suggests the neighbouring fine- grained dolerite, intercalated 

 among the quartz and felspar grains. Under the microscope the whole 

 presents the appearance of incipient melting. The felspars are cloudy 

 and altered-looking, but the twin striations are often tolerably well 

 preserved, and the grains which show them can frequently be 

 identified as the usual oligoclase of the Matopo granite, with straight, 

 or nearly straight, extinction for both sets of twin lamellae. Others, 

 ■again, can be identified with considerable confidence as microcline, 

 though the cross-hatching is only occasionally recognizable. The 

 quartz is remarkably free from inclusions. Where, however, doleritic 

 material is present, it is frequently much corroded and has minute 

 needles of^gj^iiineral which is probably hornblende penetrating its 



