122 DE. A. L. DU TOIT OX THE [vol. lxXY, 



that the band was originally a quartzite with a small amount of 

 clastic felspar. 



Owing to its hardness the zone forms the summit of Unidwendwe 

 (Indwendwa) Hill; its dip is such that it just reappears along the 

 bed of the river on the south-east, and also on the north within 

 the great bend, and the strata can be well studied here. 



The marbles resting upon this quartzose horizon appear to have 

 carried layers of chert as well as nodules of the latter, indicated 

 by bands and irregular bunches of tremolite up to a foot or so 

 across, in which the crystals are nearly an inch long ; con- 

 spicuous are little leaves of graphite. Most commonly there is a 

 transition from the marble through tremolite-rock to a core of 

 quartz ; some of the features are such as would be expected to 

 have arisen from the metamorphism of ' pre-gneiss ' quartz- or 

 chert- veins and nodules in the marble. 



When the quartz-schist band is traced out, the marble mass is 

 found to have been bent into a pitching arch, with one limb dipping 

 north-north-eastwards or north-eastwards followed by a syncline in 

 that direction, and the other at a high angle west-south-westwards 

 (tig. 2, p. 120). What are apparently the lowest zones appear on 

 F.N., the highest on Ndongini, but the original base of the group 

 was not found, the junction with the granitic complex being 

 everywhere an intrusive one. 



The marbles are almost wholly dolomitic in composition, though 

 bands low in magnesia are almost certainly present also. The 

 differences between the two groups are inconspicuous ; not far 

 above the base of the upper division are some dark-weathering 

 and finer-grained types, grey, pink, or dark red : for example, 

 a little to the north of the south-western corner beacon of 

 La Joncquet and in the south-western corner of The Forest. A 

 specimen examined was dolomitic, but contained no carbonates of 

 either iron or manganese, and the dark colour was seen under the 

 microscope to be due to minute quartz-crystals carrying dust-like 

 particles of hnematite which give a maroon tint b}' reflected light. 

 Another dark-red coarsely-crystalline dolomite carried, however, 

 some carbonate of iron. 



A second point of difference is that, near the top of the upper 

 group graphite is fnmd in layers conforming with the stratification 

 of the marbles along a thin zone crossing the river from the 

 northern end of La Joncquet to Xdongini. The mineral occurs 

 either disseminated or concentrated into bands and irregular 

 patches ; the matrix is for the greater part a yellowish calcite, 

 and, where the rock is coarsely crystalline, the graphite-Hakes are 

 larger — sometimes over a quarter of an inch across. 



Graphite is, however, not at all uncommon in the marbles, in 

 small quantity, indicating that a limited amount of carbonaceous 

 matter was originally distributed irregularly through the formation. 



The largest of the isolated marble belts is one running due 

 north from the main road on Glendale for over a mile, and dipping 

 at a very high angle westwards. There is a narrow parallel strip 



