part 1] AXD ASSOCIATED ROCKS OF MOZAMBIQUE. 



/ / 



(2) Graphic Granite and other Quartz-Felspar Rocks. 



Among the rocks distinguished as pegmatites, graphic granite is 

 hj far the most abundant. Beautiful specimens can be picked 

 up from many of the river-beds and from the gravel-strewn 

 sea-flats of the coast. In situ, the rocks occur as narrow dykes 

 and sheets from 1 inch up to about 2 feet thick. It is scarcely 

 possible to record particular localities, for graphic granite is to be 

 found in almost every square mile explored. However, in certain 

 districts the rock is more abundant than in others, and Fort 

 Jagaia, the Ampwihi Crossing, Ligonia, and the district south- 

 west of Sawa, are thus worthy of mention. 



The minerals present are quartz and orthoclase, the latter con- 

 taining (in all specimens that were sliced) drawn-out perthitic 

 inclusions of albite-oligoclase. The only other mineral visible to 

 the naked eye is muscovite, which is rarely present ; while in three 

 cases panning the crushed rock failed to give any considerable 

 residue of accessories, only a very few minute grains of magnetite, 

 zircon, and apatite being left in the pan. Of these, only magnetite 

 has been seen in thin section. 



In the field 1 applied the Rosiwal method of mineral analysis to 

 graphic granite in situ, measuring up with a millimetre-scale 

 a metre or more of smooth surface at each locality. The following 

 results (calculated to percentages by weight) were obtained : — 





Iable IX. 





Locality. 



Quartz. 



Felspar. 



Fort Jagaia 



263 

 27-9 

 253 

 25-6 

 27-1 

 24-2 



26-1 



[ 2575 

 3 



73-7* 



72-1 f 



74-7 



74-4 



72-9X 



75-8 



73-9 

 74-25 



Ampwihi Crossing 



Ligonia 



Xrassi River 



South-west of Sawa 



Chinga 



Average (by weight) . . . 



Eutectic proportions by 

 weight (after J. H. L. 

 Vogt) 





* includes about 12 per cent, albite-oligoclase") , ,, . 



f Do. do. 10 do. do. do. ( mea „!^ ed0nthm 



Do. 



do. 12 do. 



do. 



do. 



sections 



The range of variation is remarkably small, and the final 

 average agrees closely with the figures given by J. H. L. Vogt 1 for 



1 'Die Silikatschmelzlosuiigen ' pt. 2 (1904) p. 113 (Vidensk. Selsk. Skrift 

 Math.-naturv. Klasse). 



