Siimmaiy of Geological Observations. 



In some districts, as at Kail)0 ami in the iieighlxjurhood of Fort Portal, are 

 found isolated masses, or accumulations of masses, on the summits or the flanks 

 of hilly elevations consisting of laterite. In the specified districts these masses 

 consist of dialtase, and we may take it that their presence is due to the resistance 

 of certain rocks, perhaps originally in the form of dikes, and to the metamorphism 

 which reduced to laterite the gneiss or granite in which they were embedded. 

 In some places it is not improbable that it may be a question of some kind of 

 transport. 



Vulcanism. — Eecent igneous formations are met at the eastern foot of 

 Ruwenzori. Here they serve to indicate the presence of one or more lines of 

 fracture in relation A\dth that great Rift Valley with which originated the 

 depression comprising Lakes Tanganika, Kivu, Albert Edward, Albert, and the 

 Semliki Valley, and which contributed to the isolation of the Ruwenzori Range. 

 In the Fort Portal district volcanic action is indicated by thermal springs 

 (Butanuka), and by stratified tuti" which cover the ground and form a series of 

 little volcanoes, whose craters are now mostly flooded with tarns. They form a 

 chain which is disposed ver}' nearly in the direction from south to north. 



The tuffs of this formation are partly compact and partly of loose structure. 

 All, however, are of subaqueous origin, and thus attest the greater extension in 

 former times occupied by Lake Albert Edward, which must probably have been 

 united with Lake Albert towards the north. 



The compact tuffs are of a dark hue, and ver}^ hard, and yield a cement 

 consisting of a basic silicate rich in iron and easily decomposed by acids. Such 

 tuffs occur in all the craters of the series, only more or less transformed, the 

 change consisting in a tendency to acquire a red colour due to the decomposi- 

 tion of the silicate of iron. 



The tufts of looser structure, which are met partly in the craters and in all 

 the surface formations, are of a colour passing from white to grey. They derive 

 principally from fragments of the compact tuft' cemented by calcite after the 

 complete discoloration caused by the metamorjjhic process. The enclosed exotic 

 fragments are muiierous, especially in the non-compact variety, and they 

 consist of fragments of the most diverse sizes, whether water-borne or not. 

 These ingredients may for the most part be considered as coming from rocks of 

 the Ruwenzori Range, such as gneisses, diabases, diorites, garnet-bearing rocks, 

 aniphibolites, etc. In the hill at Fort Portal the tuffs are, moreover, rich in 

 vegetable remains which, unfortunately, cannot now be determined. In the 

 Butiti — Fort Portal region, mineral springs are also numerous, and the country 

 is sul)ject to frequent earthc[uakes. 



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