part 1] THE PEE-CAMBRIAN ROCKS OF MOZAMBIQUE. 7&" 



coloured felspar, measuring up to 10 feet in length, by 6 feet in 

 width, are exposed. These dykes contain small ' books ' of biotite, 

 between the folia of which green autunite 1 sometimes occurs in. 

 rosettes and irregularly-disposed flakes. This alteration -product 

 was found in many of the biotite-bearing pegmatites of other 

 localities, and also — though in much smaller quantity — in the older 

 granulitic pegmatites. No primary radioactive minerals such as 

 are elsewhere associated w T ith autunite were found, although a 

 careful search was made for any parent material from which the 

 autunite might have been derived. 



Other districts especially noteworthy for their felspathic peg- 

 matites are Mhala (Nakavala), Ribawe, and Nrassi districts 

 (map, PI. XI), and around the Mluli Mountains. But, as in 

 the case of graphic granite, it is almost invidious to choose special 

 localities, for the rocks are everywhere represented to some extent, 

 and the superficial gravels of the plateau are often thickly strewn, 

 with fresh cleavage-fragments of massive felspar. 



(3) Pegmatites with Special Minerals. 



Although the great majority of the pegmatites are composed hu 

 the aggregate of quartz and felspar with occasionally a little mica,, 

 others have been found in which hseniatite, magnetite, or ilmenite 

 are abundant constituents. 



Hsernatite occurs more particularly in the granulitic pegmatites ; 

 but at the northern end of the Chica Range, and between 

 Mount Mataria and the Mavili Hills (south-west of Sawa), large 

 rounded masses of hseniatite are also found in the later massive 

 pegmatites. 



Magnetite-bearing pegmatites are more common, and in them-, 

 the magnetite often occurs in well-shaped octahedral crystals 

 varying in axial length from 1 to 5 cm., while irregular masses 

 of larger size are also frequently present. On the slopes of East 

 Peak, Sawa, and between the Bwibwi and Sawa Rivers, such 

 magnetite-rocks are very abundant in narrow dykes and sills. 

 Despite the restricted width of the pegmatites, the crystallization 

 is very coarse, large ciystals of quartz, red perthitic orthoclase, 

 oligoclase, and biotite being developed in irregular fashion. A 

 precisely similar rock occurs at the south-eastern end of the 

 Ampwihi limestone, and its contact-phenomena with the latter 

 have been described in a previous section (p. 49). In this case 

 the magnetite was found to be titaniferous. South of Mahalia, in 

 the Malema Valley, pegmatites with large magnetite-crystals were 

 found by Mr. E. J. Starey ; and Mr. Wray has recorded the 

 presence of the same type from the districts around Wampahua 

 (Luli River) and Namkala (south-west of Mluli Mountains). In. 

 the rough rocky country south-west of the Ribawe Mountains 

 (between the Bwibwi and the headwaters of the Makwassi), 



1 See A. Holmes, ' The Age of the Earth' 1913, p. 153, for the analysis of 

 this mineral for lead and the discussion of its significance. 



