70 DE. A. HOLMES OaST THE PRE-CAMBBIAN [vol. lxxiv, 



There can be no doubt that the pegmatites are of practically 

 the same age as the granites with which they are associated. The 

 peripheral and internal quartz-orthoclase zones have a texture 

 identical with that of the tongues and apophyses of granulitic 

 granite. Although the pegmatites are generally seen cutting gneiss, 

 they may sometimes be observed, as on the lower slopes of Mhala 

 (Nakavala), as ' contemporaneous veins ' swelling and thinning in 

 irregular fashion within the larger granite masses, but without 

 definite contacts. The pegmatites are distinguished from those of. 

 later age — which occasionally cut the earlier ones — by their zonal 

 structures and granulitic textures, and by their curious mode of 

 occurrence. While the granulitic granite tongues are approxi- 

 mately of dyke- or sill-like habit, the pegmatites have often a 

 pipe-like habit of constantly varying cross-section, sometimes oval, 

 sometimes elongated, in places swelling out to a diameter of 

 several feet, and in others almost disappearing. The axis of 

 intrusion is generally more nearly vertical than horizontal, as 

 though the magma had fluxed its path upwards. Supporting this 

 view — and also indicating that the country-rock was itself 

 moderately hot — is the fact that the foliation of the gneisses 

 traversed by the pegmatites is turned upwards, so that within half 

 or a quarter of an inch from the contact, it becomes nearly 

 parallel to the outer surface of the pegmatite body. The later 

 pegmatites cut the gneisses without deviating the foliation and 

 exhibit a totally different habit, for they occur in broad dyke- or 

 sheet-like bodies that can often be traced for considerable distances, 

 both vertically and along level ground. Their minerals are either 

 graphically intergrown on a coarse scale, or are segregated in large 

 individuals, but without zonal arrangement. 



(2) Granites and Biotite-Granites. 



On the slopes near the foot of Mount Kwera (Monapo River, fig. 6, 

 p. 46) occurs a grey porphyritic granite with a fine-grained granulitic 

 ground-mass (No. 57). The phenocrysts are perthitic orthoclase, 

 while in the ground-mass microcline is the most abundant felspar, 

 the other minerals present being albite-oligoclase and quartz. No 

 accessory minerals can be seen in thin section ; but, as the result of 

 crushing and panning, a few flakes of biotite and granules of haema- 

 tite were found. In the specimen collected there are in places rusty 

 holes that were formerly occupied by haematite. A similar rock 

 (No. 63), but carrying crystals of haematite of about the size of a 

 pea, occurs in sill-like masses on the slopes of Nhepa (north-west 

 of Ribawe). In the rough country south-west of Sawa, the low 

 domes and turtlebacks are frequently thickly threaded with this 

 type of granite, and haematite masses often as big as a walnut 

 lie sprinkled about the surface. These are greatly prized by the 

 natives, who collect them for the making of spear-heads and knives. 

 The granite from Mhala, Nakavala (No. 32), is of a type similar 

 to those just described, but is free from haematite. 



