﻿Vol. 6 1.] ON ROCKS FROM THE CONGO FREE STATE. 657 



represented by chlorite. Sphene and apatite are present in 

 grains of moderate size ; magnetite is prominent in large irregular 



masses. 



No. 52 (granite-gneiss). — A medium-grained rock, showing 

 dull greenish and dull pink-stained felspar, with abundant dark 

 grains of mica. 



In sections the rock is seen to be principally made up of large 

 idiomorphic or nearly-idiomorphic felspars; the bulk of this 

 appears to be orthoclase, but some plagioclase is present. All these 

 large crystals are extremely altered, giving rise to a mixture of 

 epidote, in short stout prisms, muscovite, chlorite, zoisite, 

 and actinolite. 



Mica of a dark-greenish colour is abundant. Hornblende is 

 only doubtfully present. Epidote and leueoxene appear in 

 patches. Most of the quartz occurs as granules between the 

 felspar-xenocrysts, and it is frequently associated with a later crop 

 of felspars in clear granules. 



The above-mentioned rocks have so many points of similarity 

 that it would seem to indicate that in this large area we have to 

 deal with a massif of great uniformity ; for, although no other 

 specimens have been examined, the granite from the whole region 

 is reported to be quite similar in character. 



The District about Arebi. 



This region includes the country between the Uelle-Kimbali on 

 the east, and the Gadoa "River (a tributary of the Uelle) on the 

 west, and lies north of the Eiver Bomokandi. It includes Mounts 

 Tena, Gaima, Yagu, Tiberi, and Combe. 



The specimens from this district are Nos. 10, 11, 12, 13, 17, 

 21, 34, 35, 36, 37, 39, 40, 41, 44, 44 a, 53, & 56. 



No. 41, from the summit of Mount Tena. — A massive, finely- 

 granular iron- ore. 



No. 40, from the conglomerate at the foot of Mount Gaima.— 

 A pebble of massive, finely-granular hsematite-ore, similar to 

 material at the summit of the mountain. 



No. 10, from the top of Mount Gaima.— Massive magnetite, 

 with haematite. 



No. 11, from the top of Mount Gaima.— The rock has the 

 appearance of an irregularly-laminated (? shearing-structure) iron- 

 bearing sandstone or quartzite, weathering on the edges 

 into honeycomb-structure. 



It consists of finely-granular quartz and iron-oxides (magnetite), 

 the latter occupying a large proportion of the rock-mass. The 

 quartz-grains are very uniform in size, except in the small veins 

 which intersect the rock, the grains in these being rather larger. 



Q. J. G. S. No. 243. 2 z 



