Academy of Sciences] 

 No. 1] 



AFRICA. 



49 



stratification much more clearly. Its lower surface forms an elongated basin, and it reaches a 

 thickness of about 2,000 feet. The upper portion is a feldspathic norite of specific gravity, 2.915, 

 while it graded down into coarse-grained olivine-gabbro and " picrite," reaching a specific gravity 

 of 3.275. Banding due to the presence of layers of different mineral composition is a frequent 

 feature. Light colored, lenticular streaks of norite, free from olivine, occur conforming to the 

 gentle inward dip of the banding in the gabbro, and these as well as the gabbro and picrite are 

 traversed by occasional veins and bands of diorite and micropegmatite. The banding, when 

 present in the upper portion, is only feebly developed. Beneath the part of the picrite is a 

 wedge of gabbro, possibly injected at the base of the complex during or just after the differen- 

 tiation of the more basic lower portion. It loses its characteristic texture about three-quarters 



Fig. 15.— Differentiated intrusive masses in South Africa. (After Du Toit.) 

 A. Tabankulu mass. B and O. Insizwa mass at Brook's Nek (B) and Nolangeni Mountain (C). 

 1. Horizontal sediments. 2. Gabbro, lower part olivinic. 3. Dolerite. 

 4 and 5. Oabbro-norite poor (4) and rich (5) in olivine. 6. "Picrite." 



of a mile from the picrite, then gradually becomes ophitic in texture and may be traced into an 

 inclined sheet of coarse dolerite of the normal type which doubtless formed a feeding channel 

 for the main mass. Adjacent to this is the Tonti mass, a gigantic sill up to 2,500 feet in thick- 

 ness of gravitationally differentiated gabbro splitting into many narrower sills of dolerite (Du 

 Toit '12). The Ingeli mass, which is fed by an uprising sheet on either side, consists of a grav- 

 itationally differentiated mass of olivine-norite and picrite, the latter forming a lens-shaped 

 mass at the base of the norite which covers it to a depth of nearly 3,000 feet in places. The 

 range of specific gravities is from 2.98 in the higher parts of the mountain to 3.29 near the base, 

 where the rock closely resembles harrisite (Du Toit '13). In all three of these regions, pyrrho- 

 tite, chalcopyrite, and sometimes pentlandite occur in the basic margins. 



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