Appendix C. 



some places is accumulated hy the raiu waters in depressions of the ground. 

 Common also at varying depths are great beds of limoniti, which in some places, 

 as at Butiti, are accompanied by oxides of manganese. 



This limonite is mined by the natives, who, 1)y 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. 



]'/n/siral 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 bea considerable discordance between the archsean and palseozoic 

 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 depiived 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 moutonm'es of the 

 French, which so strangely resemble the glacial features of our lands. The fact 

 is explained liy 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 ru1)bly fragments of which get constantly displaced and washed 

 away by the rain waters. 



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

 clue 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, pi-esenting greater resistance to decomposition, end 

 by being at last completely isolated and detached. In the vicinity of ^luyongo 

 hundreds of such masses occur in the form of prisms, cubes and olielisks, at 

 times of remarkably regular outline. 



Another consequence of this predominantly superficial disintegration is 

 a peculiar ruggedness which is presented liy tlie surface of the rocks, and is due 

 to tlic pi-otrudiiig quartz that resists the decomposing forces, while the felspar 

 part is l)roken up and cairied away by the water. In the zone of the coarse- 

 grained granite this protiusion on the surface is noticed even in the case of the 

 largely hydiomorphic crystals of felspar. 



386 



