﻿654: GEOLOGY OF TART OF THE CONGO FREE STATE. [Aug. I905, 



of basic andesite appear to cut through the gneiss in a south- 

 westerly and north-easterly direction. At Redjaf, the mount of that 

 name is an isolated hill of gneiss, rising about 300 feet above the 

 level of the Nile. 



Returning from Eedjaf towards Loka, one crosses the Dinkala 

 Hills, a granitic group of some importance. From these hills to 

 Loka, bands of metamorphic rocks were observed, striking very 

 regularly north-west and south-east. 



IY. Summary. 



The Uelle district (leaving aside the small portion described as 

 forming the Rubi Basin, where the rocks are of more recent 

 formation) consists entirely of an area covered by the very shallow 

 and broad basin of the Uelle River and its tributaries. This long 

 belt is of great geological uniformity, and its whole substratum 

 appears to be granitic and metamorphic. 



The granite met at Libokwa, and also that found in the zone of 

 Bima-Bomokandi-Zobia, seems to be of normal type ; but, in the 

 absence of specimens, it is difficult for me to attempt to class it 

 definitely. As, however, at the station of Libokwa we noted an 

 isolated block of true gneiss, this would point to the rocks of the 

 whole district having undergone dynamometamorphism. 



Towards Mount Angba the country becomes distinctly and 

 entirely metamorphic in character. North of Ndoruma, as well as 

 at Mount Bundukwa, the rocks also are all metamorphic, a fact 

 which leads one to infer that the entire country lying north of 

 the Uelle, although offering to view only alluvium and limonitic 

 rocks, is close to its surface underlain by metamorphic rocks of 

 the same nature. 



Recent eruptive rocks, so far as I have been able to ascertain, 

 are apparently very rare. 



In the south-eastern mountainous portion of the Uelle district, 

 rocks of a more basic composition are as a rule prevalent, and 

 may be said to form an intrusive mass, on the margin of which 

 abut the iron-ore beds of Mounts Tena and Gaima. These beds were 

 probably upthrust by the underlying diabasic rock, as seen on Mount 

 Gaima. 



The close resemblance of the iron-ore of Mount Tena with that 

 of Mount Gaima suggests the possibility that both may have 

 belonged at one time to the same formation, subsequently disrupted 

 by the intrusive mass. 



Throughout the Uelle district the abundant argillaceous deposits 

 represent the ancient alluvial formations, and to some extent 

 also the decomposition in situ of the granitic rocks so prevalent 

 everywhere in the district. The frequent occurrence of the 

 limonitic rocks probably also bears some relation to the numerous 

 iron-ore hills existing in the district, as already pointed out. Both 

 these formations are evidently of comparatively-recent date. 



