VEINS AND [NCLUSIONS IN STELLENBOSCH GRANITE. -'4<» 



The dyke-rock referred to here is distinctly diaschistic (leuco- 

 cratic) in all its facies. whether aplitic, porphyritic, or pegmatitic. 

 Among segregation veins, however, truly aschistic types are 

 never found, and I know of no exception to the rule that segre- 

 gation veins are always more Icucocratic * than the rocks which 

 contain than. If the essential similarity of composition between 

 granite-aplite. granite-porphyry, and granite-pegmatite be ad- 

 mitted, the differences between these types are seen to be purely 

 structural, and. as such, they find a ready explanation in terms 

 of the conditions under which the molten rock solidified. It is 

 a matter of everyday chemical practice to procure a yield of 

 small crystals of uniform size from a solution by agitating it 

 during the act of crystallisation, whereas the same solution, if 

 permitted to crystallise without disturbance, will yield relatively. 

 few crystals of much larger size. A change from tranquility to 

 agitation before crystallisation is completed will produce a mixed 

 crop of large and small crystals. All the essential differences 

 between aplites, porphyries, and pegmatites can be explained by 

 such differences in the condition attending their crystallisation. 



An exceptional facies of the aplite is exhibited by a block 

 of rock which was found detached from its matrix at a spot 

 where the granite has been excavated. The bulk of this block 

 consists of an exceedingly fine-grained (o.i mm.) aggregate of 

 microcline and quartz, in which there He large sheets of biotite. 

 Each such sheet has a width which may be as much as three 

 centimetres, while the thickness of the sheet is not more than 

 half a millimetre. These sheets are not single crystals, but each 

 sheet consists of a great number of tiny scales arranged in 

 parallel position and touching or even overlapping one another. 

 There is no parallelism in the orientation of the different sheets, 

 which lie at all angles in the rock and largely determine its 

 fracture. Under the microscope the biotite is seen to include 

 grains of quartz and felspar very numerously, indicating that 

 it was formed later than these. A good deal of magnetite is pre- 

 sent in the form of rounded grains, together with a little mus- 

 covite and zircon. In the middle of this fine-grained rock there 

 is a one-inch vein of tourmaline pegmatite similar to those men- 

 tioned above. Although this block of rock was not actually found 

 in place in the granite, it seems clear that it represents an abnormal 

 development of one of the aplite-pegmatite veins, but I am at a 

 loss to account for the peculiar development of the mica. The 

 only rock in which I can recall any similar structure is the mica- 

 peridotite ( scyelite) of Caithness, Scotland, where the mica has 

 the very same habit and conditions the fracture of the rock just 

 as in this case. 



The fissure veins are narrow cracks which can be 'traced for 

 considerable distances in the granite. They are in part empty, but 

 to a large extent filled with tourmaline which has developed in the 



* The case of the nepheline pegmatites precludes one from saying 

 more acid. 



