68 Tlie Geology of the Gold Fields of British Guiana. 



oligoclase and quartz, with flakes of museovite, most of the mica being 

 original, but some of it secondary from biotite ; in some specimens 

 scattered flakes of sericite are found. In some parts of this rock the 

 himelh^i of muscovite are bent, whilst plates of feldspar and patches of 

 quartz show strain-shadows. 



With the exceptions of the aplite and the muscovite-granite of the 

 Mazaruni River district, which yielded to assay four grains of gold per 

 ton of the rock in the colony, these rocks, where unaltered, are free from 

 the precious metals ; but certain altered aplites are more or less 

 auriferous, especially, as will be described later, the altered aplite of 

 Omai, Essequibo River. 



Granite. — The largest development of true granite in the parts of 

 the colony I have visited is the mass at the junctions of the Mazaruni, 

 Cuyuni and Essequibo rivers. Granite occurs also on the lower parts 

 of the Waini River, on the Pomeroon River and on the Kuribrong River. 



These granites vary in specific gravity from 2*59 to 2 62. They 

 are grey in colour, and are usually medium to fine-textured rocks, but 

 those on the Waini and Pomeroon rivers are coarse-textured. They 

 contain relatively large irregular plates of orthoclase-feldspar in places 

 with included blebs of albite, many smaller plates of microcline and 

 somewhat abundant ones of oligoclase, — the feldspars contain some 

 inclusions of small granules of epidote and minute flakes of secondary 

 muscovite, and irregular patches of quartz, which, in some specimens, 

 show faint to marked strain-shadows, whilst others are free from them. 



The micas present are in the forms of large plates of muscovite and 

 flakes and wisps of greenish biotite — the relative proportion of 

 muscovite and biotite varying in specimens from difi'erent places. 

 The plates of muscovite in the granites from the Waini and Pomeroon 

 rivers show markedly the effects of strain, being in places much 

 distorted, and stretched into large streams of smaller plates, which 

 traverse lines of fracture in the granite. The biotite in them is of the 

 type usually present in gneissose granites. A few grains of sphene, 

 rai'ely minute crystals of zircon and some granules of iron-ore are 

 sparsely present. 



Granitite. — -The varieties of granitite have been studied with the 

 following results :— 



(a) Grey Granitite. — -This is usually very coarse-textured, in places 

 with the development of large porphyritic crystals of ortho- 

 clase, or less commonly of oligoclase. It ranges in specific 

 gravity from 2-61 to 2-74, according to the projwrtion of 

 biotite it contains. 



Under the microscope it is seen to be made up of large plates 

 of orthoclase, some of which contain flakes of muscovite, while 

 others are clouded with sericite, or are uuich saussuritised. 

 Some of the plates show a microperthitic structure and enclose 

 patches or blebs of albite or of quartz. In parts large plates 

 of microcline occur in place of orthoclase. Oligoclase occurs 

 in quantity in plates of very varying sizes, which not 

 unfrequently contain inclusions of minute prisms of epidote 



