VEINS AND INCLUSIONS IN STELLENBOSCH GRANITE. 25 I 



such specimens as have been examined. Chloritic alteration pro- 

 ducts (of biotite?) are commonly present. Although the extreme 

 examples of type (b) are so different from type (a) as described 

 above, yet such a complete series of intermediate varieties is ob- 

 served in the field that it is impossible to draw a sharp line 

 between (a) and (b), and the latter must be recognised as a 

 further stage in the alteration of the Malmesbury by the granite. 



Type (c). These xenoliths resemble type (b), but contain 

 large crystals of oligoclase. The latter are not true phenocrysts, 

 being of later origin than the base in which they lie, and are, 

 therefore, of the same nature as the eyes of many eyed-gneisses. 

 One may refer these xenoliths to the Malmesbury without unduly 

 stretching the facts, but it must be admitted that positive proof of 

 such a derivation would be exceedingly difficult to furnish, the 

 product being many steps removed from the supposed raw mate- 

 rial. 



Type (d). Xenoliths of this class are not essentially different 

 from the aplites which have been described above. Biotite is, 

 however, more abundant in the former than in the latter. The 

 grain is decimillimetre, and the texture typically granitic. I am 

 not prepared to make any suggestion as to the origin of these 

 xenoliths ; they are widely different from the others. 



Type (e). There seems to be no reason to doubt that this is 

 really an inclusion of older within younger granite. It is interesting 

 in view of the possibility that there have been two distinct periods 

 of intrusion in this area, but it would be absurd to form any conclu- 

 sion from an isolated instance. The boulder contains a little more 

 biotite than the surrounding granite ; it is also rather smaller 

 grained and has fewer phenocrysts. The average grain, exclud- 

 ing the phenocrysts, is about two millimetres. Oligoclase and mi- 

 crocline are about equally abundant, and a little muscovite is 

 present. 



Type (f). In the tourmaline xenoliths quartz and tourma- 

 line are the chief constituents. The latter mineral is brown 

 (rarely blue) in colour and does not have the radial acicular struc- 

 ture of most secondary tourmaline ; it forms stout crystals which 

 are moulded round the quartz. Fresh oligoclase and microcline 

 are present, also some large plagioclases which, though turbid 

 and decomposed, are not tourmalinised. In this instance, there- 

 fore, the tourmaline does not seem to have been formed at the 

 expense of felspar. Each xenolith of this type is surrounded, as 

 already mentioned, by a zone of large fresh felspars. These 

 xenoliths ( if they are xenoliths and not infilled drusy cavities) are 

 unique in my experience, and I am as yet quite unable to explain 

 their formation.* 



* My friend Mr. F. P. Mennell writes me that he has found entirely 

 similar inclusions in the Dartmoor granite, Devonshire, and he intends to 

 describe them shortly. It is to be hoped that he may be able to indicate 

 their origin. 



