Southern Nigeria, West Coast, Africa. D381 
This is a fine-grained porphyritie biotite-gneiss. The microscopical 
character of the mica-schists of the lower reaches of the Akpa lyefe 
need be but briefly described. One of the most typical examples 
taken from about half a mile below the Falls is distinguished by the 
abundance of a reddish-brown and reddish-yellow mica. The mineral 
forms exceedingly irregular and tattered flakes, which are always 
small. Less common than this biotite, and associated with it, are 
erystals of muscovite. Quartz and an acid plagioclase build up the 
greater part of the rock. 
Other specimens contain nearly equal quantities of biotite and 
hornblende. Quartz is locally very abundant; the plagioclase may be 
of rather a basic variety; sphene and apatite are accessory minerals. 
The pegmatoid granites associated with the mica-schists of the Akpa 
Iyefe are a well-marked group. The clearly intrusive nature of these 
rocks and the closeness of their relations to the schists, the one 
succeeding to the other without sign of chilled edges and with the 
utmost irregularity, make them of some interest. Their texture 
varies considerably ; some coarse varieties, forming ill-defined veins, 
are remarkable for their large pink orthoclases and lump-like masses 
of quartz having a pegmatoid habit. Doubtless they differ slightly in 
date of intrusion. Small brownish-red garnets, plates of muscovite 
occasionally about an inch across, and, not least important, small 
nests and crystals of black tourmaline are usual accessories. Biotite 
is found occasionally, but rarely in quantity. Except some aplite 
dykes, these coarse varieties appear to be the youngest members of 
the series, and form irregular veins in a finer-grained rock of 
essentially the same composition and appearance. 
‘The mica-schists of the Kwa River in the neighbourhood of the 
Falls below Abuton are usually reddish-brown close-textured rocks, 
remarkably well foliated. The mica is by far the most conspicuous 
mineral. In other specimens the rocks are very massive, brownish- 
black in colour, and speckled with crystals of felspar. Now and 
again hornblende-schists occur sparingly. Characteristic of the 
neighbourhood is the occurrence of a tourmaline-bearing garnetiferous 
gneiss, which is associated with the schists so closely as to produce 
a beautifully banded rock by a process apparently of lt-par-lit 
injection. This banding is sometimes on a broad scale and not very 
clear, and at others finer and exceedingly regular. Now and then 
the two rocks interlock in wedge-like forms, while the intrusive 
nature of the more acid is shown by the presence of fragments of the 
schists, more or less disintegrated, contained by it. 
The Gnewsses.—Descriptions of a few sections at and between the 
villages of Netim and Ibum on the western side of the Oban Hills 
will serve to show the nature of the gneiss of the country. 
At the first stream north of Netim on the Netim-Ibum path the 
oldest rock is a biotite-hornblende-schist rich in the ferromagnesian 
minerals, and containing some quantity of a yellowish-green con- 
stituent, probably epidote. This passes into a biotite-gneiss, or, 
1 Compare G. A. J. Cole on the production of banded gneisses by the incorporation 
of sedimentary and igneous material by an invading granitic magma (‘‘ Marginal 
Phenomena of Granite Domes’’: Rep. Brit. Assoc., 1905). 
