Great Dyke of Norite of Southern BJiodesia. 
19 
narrow continuous border around the bastite crystals, but also occurs in 
the interior of some such bordered crystals as rod-like lamellar inter- 
growths, in other parts where the rock is comparatively fine-grained (dark 
constituents one-tenth the above dimensions) and very quartzose, it 
occupies the whole crystal, the central bastite being more or less absent. 
It is probable that the presence of amphibole is due to a change in this 
residual magma, quartz and hornblende being produced instead of felspar 
and rhombic pyroxene already begun, but it might also (but less likely) be 
the result of primary pneumatolytic replacement of the rhombic pyroxene, 
the action taking place chiefly from the margin of the crystals and pro- 
ceeding inwards. The process would naturally be more complete where 
the rock is fine-grained. Subsequent to the formation of amphibole, 
however brought about, the remaining rhombic pyroxene has altered to 
bastite. 
An interesting example of these granite veins is exposed in the Little 
Tebekwe river about a mile south of the Selundi road drift. A prominent 
vein trending north and south is traceable for about two-thirds of a mile. 
It crosses the river diagonally and forms a vertical cliff on one bank for 
some distance. The vein lies in enstatitite, which is altered to a bastite 
rock for some 4 to 6 inches (less commonly to 2 feet) alongside the vein, 
probably by percolating water at the junction. Along its course the vein 
widens and narrows, in places almost dying out but widening again to 10 
or 12 feet in a few yards. Its greatest width is perhaps some 9 feet, the 
greater part of which consists of a highly jointed, fine-grained homo- 
geneous granular felspar-quartz-biotite rock — practically a granite. This 
is centrally placed. Both sides are bounded by a coarse norite pegmatite, 
respectively from 9 inches to 2 feet wide, sharply demarcated from the 
fine granitic central part in some instances, but gradually merging into it 
in others. This pegmatite consists of coarsely crystallized bastite and 
white felspar ; in places one side is coarse, the other fine-grained. At one 
end the vein consists of a foot or less of this coarse norite pegmatite, 
which rapidly expands to nearly 2 feet and becomes relatively fine-grained 
(the dark mineral is amphibole with less chloritized mica and the rock has 
the appearance of a coarse diorite). Continuing on the rock again changes, 
this time into a coarse quartz-felspar pegmatite in which little or no ferro- 
magnesian mineral is present and the quartz and felspar are partly micro- 
graphic. Succeeding this again comes the fine biotite granite which 
typically occupies the central part of the vein as above mentioned (small 
patches, spots, and streaks of the coarser norite pegmatite are contained 
in this portion of the vein in some places). Farther along it forms a 
chain of lenticles which seem to die out completely for a few yards between 
the expansions. The surrounding enstatitite contains small seams and 
