L I B R A R Y j ^ 



ON VEINS AND INCLUSIONS IN THE STELLENBQ$Cg 



GRANITE. 



By Prof. Samuel James Shand, D.Sc, Ph.D! 



Plates 4, 5, and 6. 



A series of comparative studies of the granite masses which 

 bulk so largely in the foundations of the Western Province might 

 produce new evidence regarding the early geological history of 

 the continent. The present notes can have little interest apart 

 from their relation to that larger investigation, of which they 

 may be considered as an isolated chapter. It is to be hoped that, 

 as the number of workers increases, other chapters will be com- 

 pleted from time to time and will find their places in the series. 



The large mass of granite lying to the west of Stellenbosch 

 presents few extensive exposures, but small outcrops are fairly 

 numerous. In the immediate neighbourhood of the village there 

 are three points where quarrying operations have been conducted 

 upon a small scale. The first of these is by the bridge over 

 Kromme Rivier, a short distance beyond Bosman's Crossing 

 station. Here the granite is in close proximity to the Malmes- 

 bury slates, which are met about two hundred yards up stream. 

 Micaceous inclusions and tourmaline veins are seen in the granite 

 at this point, but these are neither more numerous than, nor in 

 any respect different from, corresponding structures which occur 

 in the heart of the granite area and are described below. The 

 Malmesbury rocks are altered to a compact, splintery mica- 

 hornfels. but show no injections of granite. 



A much better exposure is met at Platte Klip, half a mile 

 to the west. This is a bare surface of granite measuring one 

 hundred and thirty by fifty paces, and exfoliating in slabs of 

 from one to two feet in thickness. A few such slabs have been 

 quarried for kerbstones. In this small exposure there is quite 

 a remarkable development of veins and inclusions, which fall 

 into the following classes : — 



I. Segregation veins of aplite and pegmatite. 



II. Fissure veins with infilling of tourmaline. 



III. Inclusions (xenoliths) of various types. 



(a) Micaceous xenoliths of very dark colour and deci- 

 millimetre grain (i.e., average size of grain 0.1 to 

 1.0 millimetre). 

 (b) Micaceous xenoliths of coarser grain (1 to 2 mm.). 



(c) Similar to( b), but containing large crystals of red 

 felspar. 



(d) Fine-grained xenoliths resembling microgranite 

 and containing more felspar than mica. 



(e) Granite xenoliths. A single boulder, some nine 

 feet in diameter, appears to be an inclusion of 

 earlier within the later granite. Its relation to the 

 latter is clearly shown in photograph No. 1. 



