Descriptive Geology. 107 



quartz showing strain -effects ; with small flakes of greenish biotite and 

 a few granules of epidote scattered through the mass. 



HornhUnde-granitite-gnsiss. — A granitic aggregate of feldspar, 

 some of which is oligoclase, and of quartz showing strain-effects, with 

 small irregular plates of pale-blue hornblende, some flakes of bi'own 

 biotite and a few of muscovite ; a few granules of epidote, and some 

 grains of sphene and of melanite form unimportant accessories. The 

 larger plates of feldspar consist of orthoclase, and are more or less 

 sericitised. 



Gneissof^e-granite. — This rock consists of large microperthitic plates 

 of alkali-feldspar, which in places are clear and show the structure of 

 microcline, whilst in others they are ci'owded with sericite or traversed 

 by thin veins of secondary muscovite ; plates of oligoclase, and ii-regular 

 patches of granulitic quartz with small flakes of more or less chloritised 

 biotite. 



The sjjecimens of basic rocks are hornblende-schist of the following 

 types :— 



1. A dark-coloured, rather fine-grained rock consisting of small 

 plates of pale-blue hornblende, lying with their longer diameters 

 pai'allel to one another in a micro-mosaic of grains of water- 

 clear feldspar, with some of well-defined plagioclase, through 

 which are scattered a few minute gi'ains of epidote and of sphene 

 of the leucoxene-type. 



2. A hornblende-schist of more compact structure than the fore- 



going, made up of very abundant angular grains of pale- 

 blue hornblende in a micro-mosaic of grains of water-clear 

 feldspar. 



The Aruka River. — Large areas in the northern parts of the North- 

 western district are very low-lying, and are occupied by swamps. A 

 range of low hills, probably nowhere more than 1 50 feet high, runs on 

 the northern side of the Aruka River, approximately parallel to it, 

 from about 3A- miles S.W. of Morawhanna for a distance of about 17 

 miles. A few low rounded hills occur near its southern bank. The 

 hills consist either of hornblende-schist, as at Maburima and near Youpu 

 Creek, or of a massive epidiorite as at Issorora. The plain from which 

 this range rises is a part of the gneissose complex of the Guianas, and 

 specimens of its rocks have been obtained from near the Lower Maburima 

 Creek, from a small island in the Aruka, and, as already described, 

 from the Amacura River. 



The epidiorite and the hornblende-schist contain gold in small 

 quantities, varying from traces to 34 grains per ton of the rock, the 

 mean contents of the samples examined being 9-6 grains. Unfor- 

 tunately the topography of this district is not favourable for the 

 formation of placer-deposits, which, if they occurred in the vicinity 

 of these hills, should be rich ; but colours of gold have been found 

 in the gravels of the Maburima and the Araua Creeks. 



