CONTRIBUTIONS TO THE GEOLOGY OF J3ENGUELLA. 515 



far. Six miles from the Cutato the track crosses the Canji River, beside which are 

 outcrops of schistose quartzites and a coarse biotite-gneiss striking almost due east- 

 ward. Some of the gneiss about this locality contains but little quartz. The banks 

 of a rushing stream, about 4 miles north-north-east of the Canji, consist of a white 

 granitoid gneiss in which the foliation is indefinite, but long inclusions show that the 

 direction is to 83°. 



About 2 miles further on are exposures of a gray gneiss ; this rock continues 

 to the Okanusi Pass, by which the route leads down to the American Mission Station 

 at Ochilesa. Half an -hour before reaching this village I noticed the first exposure 

 of a light-coloured pinkish gneiss very rich in felspar. Mr Tyrrell's determination 

 of the ferro-magnesian constituents in this rock as hypersthene shows that the rock 

 is a charnockite. 



F. Ochilesa to Sacccmjimba. 



Ochilesa is a locality of especial geological interest, owing to its sodalite-syenites 

 and alkaline dykes ; but we had only time to spend a day and a half there, when 

 we enjoyed the kind hospitality of Dr Woodside and Mr Niepp. Along the banks 

 of the stream, the Quime, are a series of warm alkaline springs of which the maximum 

 temperature is reported as 114° F. To the south of Ochilesa is a flat-floored basin, 

 the Changa, which is about 8 miles long and 4 miles wide and surrounded by 

 an elliptical ring of hills. The proximity of this oval depression to the hot springs 

 had led to the view that the basin is a volcanic crater, and the neighbourhood of 

 Ochilesa has been described as the volcanic district of Bihe. I was unable, however, 

 to find here any evidence of recent volcanic action, though shonkinite and orrachitite 

 dykes occur on the banks of the Quime. 



The Changa basin is a cauldron due to subsidence, and the hot springs arise along 

 the northern continuation of the fault that passes along the western side of this 

 basin. The alkaline nature of the water is doubtless explained by the richness in 

 alkalies of the adjacent rocks. As the Quime is fed by these springs, it deposits 

 along its course a series of beautiful tufa terraces built up by the constructive water- 

 falls which connect the successive basins. The following analysis of the tufa, by 

 Mr W. C. Scott of Sandusky, Ohio, was shown me by Mr Niepp : — 



Silica 2*6 per cent. 



Iron and alumina . . . . .3*6 ,, 



Carbonate of lime . .... 89*64 ,, 



Magnesia ...... 2'12 ,, 



97*96 



These springs are similar to the group of Katanga hot springs described by 

 Mathieu (1913, p. 124), in which the water has a temperature of from 70° to 100° F., 

 and deposits calcareous tufa. The thermal springs in Katanga discharge at the 



