PQ,tro(jrapliy of tlie Fandamentcd Gn"Asso>te Complex. 31 



marked foliation, this showins; that the gneiss has been derived from 

 sranitic rocks, and that the foliation is a result of their exposure to 

 dynamic or regional metamorphism. 



The most usual colour of the gneiss is light-grey, but in parts it not 

 unfrequently has a pinkish tinge. In the banded varieties layers of 

 white, greyish-white, or of pink aplite-gneiss or granulite alternate 

 with layers of dark-coloured, to almost black, hornblende-granitite-gneiss 

 or diorite-gneiss, or with greenish-grey ones of epidote-granitite-gueiss. 



A large number of specimens of the different kinds of gneiss have been 

 collected from various districts and examined. The following are 

 descriptions of some of the more characteristic kinds : — ■ 



Aplite-gneiss. — This rock is of either a white or a pinkish colour, 

 and usually is fine-grained, with in places some larger crystals of 

 feldspar. It occurs in two forms — granitoidal and granulitic. Its 

 specific gravity varies from 2-58 to 2-72. The rock is generally banded 

 with hornblende-gneiss. 



The gi'anitoidal variety has been found at " Little Kamaria" Rapids, 

 and at '• Devil's Hole '' in the Cuyuni River, at Kabowira Rapid on 

 the Mazaruni River, and at Itaka on the Lower Esset[uibo Ptiver. 



The rocks consist of granitic aggregates of quartz, in places showing 

 strain-shadows, or being more less granulitic with orthoclase-feldspar 

 in places in large plates, which occasionally show a microperthite 

 structure, and often are more or less sericitised, irregularly bounded 

 plates of clear microcline, plates of oligoclase and of albite, frequently 

 with bent or broken lamellpe, and occasionally patches of vermicular 

 micropegniatite ; some flakes of muscovite, both original and secondary, 

 and small flakes of greenish biotite, are occasionally present ; minute 

 granules of pyroxene, or })lates of green hornblende occur, though 

 seldom, and only in very small ([uantities as accessories; granules of 

 epidote, of sphene, of garnet and of magnetite may be present, while 

 a few minute crystals of apatite are seen in places. 



The granulitic varieties are made n^ of a micro:iiosaic of quartz and 

 water-clear feldspar, with here and thei'e plates of oligoclase, with some 

 scattered minute granules of epidote and some grains of magnetite. 



GRANITITE-GNEISS. 



This is the prevalent gneissose rock of the colony. It varies from 

 a somewhat coarsely foliated, almost granitoidal, rock to a finely foliated, 

 almost schistose one. According to the minute structure the granitite- 

 gneiss may be classed as : — ■ 



1. Granitoidal. 



2. Commencing Granulitic. 



3. Granulitic. 



The rock consists essentially of quartz, orthoclase, microcline, 

 oligoclase and biotite. It varies in colour from light-grey to light- 

 pink, but in places has a somewhat greenish tinge. The average 



