Appendix C. 



some places is accumulated liy the rain waters in depressioiis of the ground. 

 Common also at xarving depths are great l)eds of Jimmilr, whidi in some places, 

 as at Butiti, are accompanied by oxides of mtinf/arKve. 



This limonite is mined liy the natives, who, liy the C'atalunian process, 

 extract from it the iron which they use in the mamifacture of arms and 

 implements, an industry in which they display much skill. 



Phmml 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 (juite evident 

 that there must be a considerable discordance between thearchiean and paleozoic 

 formations. 



Erodun. — Amongst the phenomena of erosion, which, as may be easily 

 understood, are very marked, mention should l»e made of the denudation, thanks 

 to which the older rocks, Ijeing deprived of their laterite covering, become 

 exposed on the surface of the ground. They usually assume the characteristic 

 aspect of mammiform oi' hummocky rocks, the so-called roehes iiniiifonnees 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 rock.s, instead of becoming disintegrated, suffer only a surface change 

 and rupture, the rul)bly fragments of which get constantly displaced and washed 

 awaj' by the rain waters. 



Outwardly the rocks often present a crust of \arying thickness, which is 

 due to metaraorphism, 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, pi'esenting greater resistance to decomposition, end 

 liy 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 (piartz 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 hydiomor-phic crystals of felspar. 



386 



