14 F. P. Mennell — Basic Dykes and Bock Genesis. 



[1702.] This specimen has a dark-grey crystalline ground-mass, 

 with enclosed patches of quartz and felspar, and although the rock it 

 represents forms large masses near the edge of the dyke, it is evident 

 they are to all intents and purposes partly ahsorbed granite xenoliths. 

 It is remarkable how little basic material can be definitely identified 

 under the microscope, though the dark colour of the rock is strong 

 evidence of its presence. The ground-mass is made up of fine- to 

 medium-grained micropegmatite with minute scattered biotite flakes. 

 The cloudy felspars derived from the granite are frequently in 

 crystallographic continuity with those of the surrounding intergrowths, 

 and these at times show the microcline cross-hatching. Zoning can 

 sometimes be observed, a wave of extinction starting at the outer 

 edge of the cloudy felspar, and passing gradually across the micro- 

 pegmatite fringe. The nucleus itself is free from the zoning, as are 

 the felspars of the surrounding granite mass. A. well-developed 

 crystal of orthite, about i- - mm. in length, is present in this 

 slice, and shows very little alteration. Slice [1704] shows altered 

 crystals of sphene, which, like the orthite, is a constant accessory in 

 the granite. It may be remarked that slices 1702 and 1704 would 

 both be taken to represent fairly typical granophyres, in Rosenbusch's 

 sense of the term, without a knowledge of their field relations. 

 (See Pig. 5.) 



[1705.] This is made up almost entirely of granitic materials. 

 Indeed, the ground-mass really shows no traceable doleritic materials. 

 It is quite subordinate in amount to the xenocrysts, or phenocrysts, or 

 whatever they are most correctly to be termed, and chiefly consists of 

 reaction zones between the quartz and corroded felspars, which afford 

 beautiful examples of micropegmatite (see Fig. 6). Apatite, sphene, 

 and orthite are among the constituents of the rock as in the surrounding 

 granite mass. Biotite is freely distributed in aggregates of small 

 flakes, but neither in this case nor in any of the other specimens does 

 it occur as actual xenocrysts. The original crystals have evidently 

 always been melted down, or otherwise destroyed, and it has then 

 crystallized out afresh amongst the ground-mass. Before closing, it 

 may be well to remark that in no case has micropegmatite been found 

 in any but marginal modifications of the Matopo granite, and in these 

 latter it is always of the vermicular type which seems characteristic 

 of rather rapid cooling under very deep-seated conditions. It was the 

 very fact of finding, among the specimens sliced from this locality, 

 20 miles from the edge of the granite, what appeared to be a by 

 no means deep-seated type of granophyre with normal micro- 

 pegmatite, that led me to re-examine the spot with care the next time 

 I travelled by. 



General Conclusions. The phenomena, which I am afraid are only 

 scantily and imperfectly described above, are in some respects closely 

 similar to those which Professor Judd recorded 1 in certain cases which 

 seemed to him to prove the secondary origin of micropegmatite and 

 the growth of crystals in rocks after their solidification. Thus the 

 cloudy central parts of the micropegmatite masses which he observed 



1 Q.J.G.S., May, 1889, pp. 175-86. 



