54 DE. A. HOLMES ON THE PKE-CAMBKIAN [vol. lxXlV, 



No. 49, from the Ibrahimo -Mosuril road, 3 miles from Ibrahimo, is a fine- 

 grained, finely-banded gneiss, in which the foliation- planes are speckled with 

 minute flakes of biotite that give to 'the rock in certain directions a charac- 

 teristic ' pepper-and-salt ' appearance. Under the microscope the minerals 

 present are quartz, orthoclase (generally altered along cracks and cleavage- 

 planes to sericite), microcline, oligoclase (holding secondary grains of epidote), 

 biotite. and a little primary muscovite. Accessories are magnetite with rare 

 examples of minute apatite and zircon. The rock has not suffered from 

 granulation, and the minerals are free from strain- shadows. Either it 

 received its foliation during crystallization, or it has been completely 

 recrystallized since solidification. (See Table V, p. 53 and PI. IX, fig. 1.) 



No. 42 occurs immediately north of Murimitigri, where it alternates 

 with quartz-mica-schists (see description of No. 172, on p. 45). The rock 

 is almost identical with No. 49 in the hand-specimen, but in thin section it is 

 seen to have the appearance of a micrographic granite that has crystallized 

 from a viscous magma flowing under conditions of differential pressure. Long- 

 tongues of quartz, giving sharp extinction, alternate with a granular mosaic 

 of microcline, of which the average grain is 2 mm. Neighbouring tongues 

 of quartz extinguish together, or in some cases nearly together ; but the 

 microcline individuals show no such relationship. Evidently, the quartz was 

 drawn out before its crystallization was completed. No. 172 shows the 

 strikingly different effect produced by the same processes acting on a rock 

 already in the solid state. 



No. 54, from 2 miles west of Mitikiti, is another rock similar to No. 49 in 

 the hand-specimen. It is, however, coarser in grain (average 4 mm.) and is 

 richer in biotite. This rock is extensively riddled by narrow pegmatite- 

 veins which make with it a composite gneiss. Similar rocks occur extensively 

 around Yenyiga (No. J+3). 



No. 51, also from the neighbourhood of Ibrahimo (1^ miles beyond Ibrahimo 

 on the road towards Mount Kwera), illustrates the more coarsely-banded varie- 

 ties of gneiss. Bands averaging a centimetre in width and composed mainly of 

 quartz, microcline, and oligoclase, alternate with narrower bands in which 

 biotite is the chief constituent. Muscovite is an accessory, and magnetite and 

 apatite are also present. The microscopic texture is nearly granitic, differing- 

 only by the presence of occasional granulitic patches. (See Table V, p. 53.) 



Nos. 41 Sf 53, from the Makwassi River, near its junction with the Bwibwi 

 at the northern end of the Mtupa Pass, Ribawe Mountains, are still more 

 coarsely-grained rocks, containing dark-green hornblende, and ilmenite asso- 

 ciated with the biotite. The biotite-gneiss near the Ampwihi Crossing is 

 an almost identical rock. No. 53 merges towards the ' augen ' type by the 

 swelling of the biotitic bands around islands of quartz -felspar aggregates. 



No. 45, from the Bwibwi Valley, below the Namosa Hills, is a gneiss with 

 bands excessively rich in deep-brown biotite. The mineral compositions 

 of the micaceous and felspathic bands respectively are set forth in Table V 

 (45 a & 45 b). The rock is quite fresh and, as in the case of most of the 

 gneisses, the structure is slightly granulitic, while the biotites are elongated 

 rather more than is usual in granites. 



No. 69 is the best example of an ' augen-gneiss.' It occurs in the bed 

 of the Ligonia River, opposite Eort Ligonia, and is a handsome rock, con- 

 sisting of large pink felspathic ' eyes ' and lenticles separated by dark-green 

 streaks of biotite aggregates. Under the microscope the texture is distinctly 

 seen to be more granular than are the banded varieties. The biotite aggre- 

 gates are of a sage-green tint, and are associated with canary-yellow epidote 

 granules, and rounded crystals of greenish-brown sphene. The epidote appears 

 to be derived fiom a calcic plagioclase with oligoclase as the complementary 

 product ; for oligoclase crystals are abundant in the neighbourhood of the 

 biotite streaks, and are invariably occupied' by innumerable grains of similar 

 epidote accompanied by wisps of muscovite. Orthoclase and microcline, 

 in addition to quartz, make up the larger lenticles and ' eyes,' and are 

 usually free from alteration, only a little secondary muscovite being occa- 

 sionally present. (See Table V, p. 53.) 



