72 The Geology of the Gold FieUh of Britisih Guiana. 



occasionally a few blebs of i>aniet, and small prisms of apatite, which are 

 usually sparsely distributed but are present in some specimens in relative 

 abundance. Small crystals of pyrite, and plates of carbonates are 

 present in parts of the rock as secondary constituents. 



Diorite. — Diorite, other than as local modifications of the granitic 

 masses, is of somewhat rare occurrence in British Guiana. Hornblende- 

 plagioclase rocks not infi'equently occur, but examination of thin slices 

 show that the great majority of them are amphil^olised diabase or 

 gabbro. The true diorites occur, as a rule, in narrow dykes and tongues 

 intrusive through the gneiss. 



They are dark-grey rocks, occasionally of coarse texture, but, as a 

 rule, fine-grained, and they vary in specific gravity from 2'S5 to 3-07. 

 The majority of them are granitic aggregates of a basic labradorite, in 

 laths and in more or less idiomorphic plates, some showing a zonal 

 structure, and usually containing inoi'e or less epidote and sericite ; a 

 very little interstitial quartz ; and abundant masses of gi'een, olive-green, 

 or pale-brown hoi'nblende, in places containing some small extruded 

 grains of magnetite, with a few granules of epidote. In some specimens 

 small kernels of pyroxene are recognisable in the masses of hornblende. 



In a few specimens the hornblende is pale-blue, and has a poikilitic 

 to an ophitic structure ; the labradorite being in laths, and here and 

 there in small phenocrysts. 



The accessory minerals in the diorite are the same as those found in 

 the quartz-diorites, and are similarly irregularly distributed. 



A small island a little below Sapira Cataracts in the Mazaruni Kiver, 

 is made u\i of a p(jrphyritic rock containing very abundant large crystals 

 of hornblende in a scanty matrix of feldspar. Its specific gravity is 

 2-97. It contains large phenocrysts of green hornblende, with here and 

 there prisms cf epidote, some flakes of chloritised biotite, and a few 

 scattered grains of magnetite set in a holocrystalline matrix of a basic 

 labradorite clouded with sericite, and, with here and there, a few 

 granules of secondary quartz. 



In the Sororieng Channel of the Mazaruni River, a dark-grey rock 

 of medium texture, and having a specific gravity of 2"89, occurs. It is 

 made up of large phenocrysts of a coloui'less augite, with peripheral 

 green hornblende, and in places with a little chlorite, some of the pheno- 

 crysts showing many extruded grains of magnetite ; some plates of dark 

 olive-green to brown original hornblende, and some flakes of brown 

 biotite ; abundant laths of labradorite with corroded edges and with 

 many included small prisms of zoisite, the feldspar being in places more 

 or less sericitised ; a little interstitial quartz ; and some scattered grains 

 of magnetite. This rock is an augite-diorite. 



Syenite. — Of even rarer occurrence than the true diorites are the 

 syenites. I have found two kinds of these, — true syenite or hornblende- 

 syenite, and augite-syenite. The only mass of the former I have seen 

 is at Pigeon Island in the Lower Essequibo River. It is a medium to 

 coarse-textured granitic rock, with conspicuous crystals of hornblende 

 in a feldspaihic matrix, and it has a specific gravity of 2-85. Under 



