Appendix C. 
some places is accumulated by the rain waters in depressions of the ground. 
Common also at varying depths are great beds of limonite, which in some places, 
as at Butiti, are accompanied by oxides of manganese. 
This limonite is mined by the natives, who, by the Catalonian process, 
extract from it the iron which they use in the manufacture of arms and 
implements, an industry in which they display much skill. 
Physical Features .—Owing to the abundant herbaceous vegetation, and to 
the thick deposits of laterite and concretionary limonite, which everywhere 
cover the ground, I was unable to make any conclusive stratigraphic observa¬ 
tions. Nevertheless, from the data which we collected it seems quite evident 
that there must be a considerable discordance between the archsean and paleozoic 
formations. 
Erosion .—Amongst the phenomena of erosion, which, as may be easily 
understood, are very marked, mention should be made of the denudation, thanks 
to which the older rocks, being deprived of their laterite covering, become 
exposed on the surface of the ground. They usually assume the characteristic 
aspect of mammiform or hummocky rocks, the so-called roches moutonnees of the 
French, which so strangely resemble the glacial features of our lands. The fact 
is explained by the absence of the factor of frost and thaw, in consequence of 
which the rocks, instead of becoming disintegrated, suffer only a surface change 
and rupture, the rubbly fragments of which get constantly displaced and washed 
away by the rain waters. 
Outwardly the rocks often present a crust of varying thickness, which is 
due to metamorphism, and this crust adheres in the loosest way to the under¬ 
lying mass. Not seldom the adherence fails altogether, and then the transformed 
surface forms slabs with rounded edges merely resting on the underlying rock, 
which is still relatively intact. 
This phenomenon is seen where the gneisses crop out. In the case of 
granites there is further noticed a cleavage of the rock in great blocks of 
parallelopiped form which, presenting greater resistance to decomposition, end 
by being at last completely isolated and detached. In the vicinity of Muyongo 
hundreds of such masses occur in the form of prisms, cubes and obelisks, at 
times of remarkably regular outline. 
Another consequence of this predominantly superficial disintegration is 
a peculiar ruggedness which is presented by the surface of the rocks, and is due 
to the protruding quartz that resists the decomposing forces, while the felspar 
part is broken up and carried away by the water. In the zone of the coarse¬ 
grained granite this protrusion on the surface is noticed even in the case of the 
largely hydiomorphic crystals of felspar. 
386 
