CHAPTEK VIII. 



THE PETROGRAPHY OF THE GRANITIC ROCKS. 



The rocks of this type occurring in the colony are aplite, muscovite- 

 granite, granite, granitite, hornblende-granitite, augite-granitite, quartz- 

 diorite, quartz-mica-diorite, diorite, syenite, augite-syenite, mica-gabbro, 

 gabbro and norite. 



These show to a greater or less extent a gneissose structure, their 

 component minerals in places exhibiting strain-eifects. 



Aplite and Muscovite-granite. — The veins of granitic rocks, which 

 in many places traverse the basal complex of British Guiana, are more 

 frequently aplite or muscovite-granite than true granite or granitite ; 

 for in them ferro-magnesian minerals are present in only very small 

 proportions to the quartz and feldspar, or in many cases are practically 

 absent. Rocks of this nature have been found in every j^art of the 

 colony I have examined ; the}^ appear to be more abundant in the 

 lower reaches of the Cuyuni River than elsewhere. 



Aplite. — Rocks of this class usually vary in colour from white oi' 

 pinkish-white to pinkish-red, but some are of various shades of grey. 

 They vary in specific gravity from 2 '5 9 to 2*68. In hand-specimens 

 some of them show blebs of quartz and crystals of feldspar, while others 

 are uniformlj^ fine-grained. In a few specimens small crystals of red and 

 of black garnet (melanite) have been found. Under the microscope the 

 aplites are seen to be granitic aggregates of plates of orthoclase, microline, 

 albite. some of oligoclase and irregular areas of quartz, whilst in places 

 they contain small quantities of quartz and feldspar in micro-pegmatitic 

 relationship to one another. Minute granules of crystals of epidote or of 

 zoisite, and leaves and flakes of sericite, are occasionally present in the 

 feldspar. Many specimens of the aplites are practically free fi'om mica, 

 but a few small flakes of brown to gi'eenish-brown biotite, of chlorite 

 after biotite, and of muscovite, are present in some of them in un- 

 important quantities. Minute grains of magnetite, of titaniferous iron- 

 ore with leucoxene, and of sjshene, and very minute prisms of apatite 

 and of zircon, are present in exceedingly small proportions as accessories. 

 In some parts the quartz shows strain-shadows, and the lamellfe of 

 some of the feldspars are bent or broken. The aplites frequently are 

 traversed by very thin veins of epidote. An aplite from Sapira 

 Cataracts on the Mazaruni River contains, in addition to feldspar and 

 quartz, blebs and patches of colourless glass, containing many microliths. 

 The glass in places has eaten into the feldspar. 



Muscovite-granite. — Wherever this occurs it is a fine-grained M-hite 

 rock of specific gravity varying from 2-59 to 2-66. It is composed of 

 an irregular granitic aggregate of orthoclase, microcline, albite, some 



