Summary of Geological Observations. 
In some districts, as at Kaibo and in the neighbourhood 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 diabase, 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 .—Recent 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 with 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 tuff 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 very 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 probabty have been 
united with Lake Albert towards the north. 
The compact tuffs are of a dark hue, and very 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, oidy 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 tuffs 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 tuff cemented by calc-ite after the 
complete discoloration caused by the metamorphic process. The enclosed exotic 
fragments are numerous, 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, 
amphibolites, 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 subject to frequent earthquakes. 
387 
2 c 2 
