Vol. 51.] ON THE GEOLOGY OF MOUNT RUWENZORI. 675 



(5) Conclusions as to the Structure of Ruwenzori. 



After this description of the facts observed it is advisable to 

 consider the evidence which they afford as to the character of the 

 mountain. There are three rival explanations. The first view is 

 that it is volcanic; the second, that of Stuhlmann, is that it is a 

 plutonic massif of diorite ; to which we add a third, that it is 

 possibly a ' block-mountain.' 



We do not presume to be able to give a final solution to this 

 problem, for it is possible that in the heart of the ridge there may 

 be rocks different from any yet found. The volcanic theory, how- 

 ever, may be at once dismissed, and the evidence is equally con- 

 clusive that Ruwenzori is not a simple massif of diorite. This 

 rock apparently occurs only as banded sheets in the schists, and is 

 not the central rock of the ridge. The plutonic is, nevertheless, 

 a possible theory. 



The strike of the schists on the flanks of the mountain seems to 

 run concentrically round the ridge. This certainly looks as if the 

 central rock were intrusive into the schists. 



The highest rock as yet collected is not sufficiently definite to 

 prove either one theory or the other. It is a coarse-grained 

 felspathic rock [140], which consists mainly of a large crystal of 

 schillerized orthoclase, and contains some quartz and microcline. 

 The rock is either a granite or a granitoid gneiss, probably the 

 latter. It was doubtless originally an igneous rock, and may owe 

 the foliation which probably occurs in it, either to fluxion during 

 cooling (as in the case of intrusive gneisses) or to dynamo-meta- 

 morphism. It may be, therefore, either an intrusive igneous rock, 

 or a part of the old Archoean series faulted up. There is nothing 

 in the microscopic characters of this rock to separate it from those 

 of the Archaean series. 



Unless further investigation bring to light other rocks in the 

 centre of the ridge/we think it probable that this rock will be 

 proved to have been raised into its present position by faulting. 

 In this case Kuwenzori is simply composed of an orographic block 

 or ' scholl.' 



III. The Giaciation oe Rtjwenzori. 1 



The Yeria, Msonje, and "Wimi valleys have the ordinary V shape 

 of erosion-valleys in a tropical and rainy climate, but the Mubuku 

 and Nyamwamba valleys are quite different. They are flat and 

 broad, with the open U shape which is most usual in glacier-formed 

 valleys. Another valley (the Butagu) on the western side is neither 

 one nor the other, but has the general shape of the latter, with a 

 deep ditch at the bottom. 



It seemed to me afterwards that this rather pointed to the 

 Nyamwamba, Mubuku, and Butagu valleys having been at one 



1 This part is contributed solely by Mr. Scott Elliot. 



