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PROF. S. J. SHAND ON 



[vol. lxxii, 



felsitic appearance. Slices cut through these red felspars show 

 that the whole substance is minutely granulated, and that in part 

 the felspar has been fused and has afterwards crystallized in the 

 form of perfect spherulites. 



It is remarkable that only the quartz and felspars of the granite 

 appear among the inclusions ; in no single instance is the biotite 

 preserved. In view of the evidence of high temperature shown by 

 the melted felspar, this becomes intelligible : biotite is decomposed 

 by moderate heat, and its decomposition has furnished the abundant 

 magnetite of the base of the rock. Although felspar is more 

 abundant than quartz in the granite, yet among the inclusions 

 quartz is more noticeable than felspar. In one case an inclusion of 

 amphibolite was discovered in one of the veins. The bright-green 

 hornblende shows no granulation, and is quite unchanged save at 

 the very edge of the inclusion, where it loses its well-developed 

 cleavage and takes on a fibrous appearance, although keeping its 

 pleochroism and its uniform extinction. Round about the inclu- 

 sion the pseudotachylyte is coloured green, hence solution is again 

 indicated. 



The only other xenocrysts that I have observed are a few 

 unchanged crystals of zircon, sphene, and magnetite. The pseudo- 

 tachylyte is always perfectly fresh, and no hydrous secondary 

 minerals have been observed in any case. 



iy. The Case for the Igneous Origin of the Veins. 



The evidence which suggests an igneous origin for these veins, 

 and led to their being regarded as tachylytes in the first instance, 

 may now be summarized : — 



(1) The mode of occurrence in the field, as described above — which is 



quite consistent with an igneoiis intrusive origin, but difficult to 

 reconcile with any other known manner of formation. 



(2) The abrupt contacts between vein-rock and granite, and the absence 



of shearing in the granite. 



(3) The common occurrence of blind intrusions. 



(4) The presence of rounded boulders and fragments in the veins, as also 



the microscopic evidence of corrosion and even fusion of these and 

 of selective destruction of the constituents in a definite order. 



(5) The presence of spherulitic and microlitic structures that find their 



nearest analogues in vitreous dyke-rocks and lavas in which crystal- 

 lization has taken place subsequent to consolidation as glass. 



(6) The sharp distinction which exists between the inclusions and the 



matrix. 



(7) Although unquestionable igneous intrusions are not often seen in the 



granite, yet there is at least one basic dyke of the largest size in 

 the very heart of the area, thus demonstrating the presence of basic 

 magma in depth. This is a dyke of granophyric quartz-dolerite, 

 nearly 600 yards wide, which is exposed in a shallow valley at 

 the weir, above Parijs. Mr. J. W. Penny has described basic 

 intrusions in the Lower Witwatersrand Beds which overlie the 

 granite. 



