﻿Vol. 6 1.] ON EOCKS FROM THE CONGO EllEE STATE. 661 



manner is an altered an gite, in ]arge masses, generally with bright 

 green actinolitic reaction-rims. Large masses of iron -ore are 

 present; they are usually surrounded by a zone of granular garnet 

 which, in its turn, is often bounded by a fringe of hornblende. 

 Occasional grains of epidote are present, as well as flakes of 

 hsematite. 



Kos. 15 & 16, between the Rivers Kwado and Aro (microcline- 

 gneiss). — These are two similar fine-grained gneisses. Mierocline 

 in abundant large plates is the predominant felspar; there is also 

 some plagioclase. Quartz occurs in irregular grains and as 

 bleb-like inclusions in the mierocline; fluid-inclusions are common in 

 the quartz. Both a colourless and a dark-brown mica are present. 

 Epidote is plentiful in rather large pale masses. Zircon and 

 sphene are present ; also small patches of chlorite in association 

 with the mica. 



The foliation is well marked, even in the slide, and is accom- 

 panied by an approach to granulation and a sort of mortar- 

 structure. (See PI. XLIII, fig. 1.) 



Hocks from the Northern Boundary of the Congo, 

 bordering the Bahr-el-Ghazal. 



The specimens brought from this region were all collected in the 

 mountain-group comprising Mount Baghinze, Mount Bundukwa, 

 Mount Nangoro, and Mount Yambili. 



Nos. 1-6, from Mount Bundukwa.- — All these specimens are 

 varieties of a very coarse pegmatoid gneiss, the several elements 

 tending to run in distinct layers and veins. Tourmaline in 

 stout prisms, kyanite in broad blades, and red garnets are very 

 abundant in some layers ; in seme places a sort of tourmaline- 

 rock is produced by the concentration of this mineral. Fairly 

 large zircon-crystals have been noticed during the microscopic 

 examination of the rock. 



Rocks from the Lado Enclave. 



Three specimens have been examined from the River Kidju, at a 

 point on the road midway between Loka and Kodjokadji — Nos. 31, 

 32, & 33. 



Nos. 31 & 32 (microcline-gneiss). — A cream-coloured, medium- 

 grained banded rock. The texture, as seen in the section, is 

 granitic ; there is no sign of foliation. Quartz occurs in fair-sized 

 plates, also in blebs within the felspar (corrosion-quartz). Streaks 

 of dusty inclusions are common. The felspars comprise abundant 

 mierocline, orthoclase, and plagioclase (oligoclase); albitic and peri- 

 clinic twinning frequently occur together. Some of the felspars 

 have schiller-structure, produced by rows of negative crystals. 

 Micrographic intergrowths of felspar and quartz appear sporadi- 

 cally in small patches. A curious, dark-green, pleochroic variety 



