The Petrogra'pliy of the Granitic Rocks. 73 



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

 orthoclase, of microcline, and some of oligoclase, a few of the latter are 

 crowded with microliths of epidote; with a very little quartz; and many 

 large, more or less idiomorj^hic plates of green hornblende. A few 

 blebs of garnet, some small crystals and granules of sphene, and a little 

 limonite are unimportant accessories. 



Augite-syenite. — Augite-syenite,moreor less altered, occurs in a broad 

 belt at the Makari Rapids, and in a narrow dyke at the Caburi Cataracts 

 on the Mazaruni River. The Makari rock consists of a porphyritic variety 

 with large phenocrysts of orthoclase, some of which are one and a half 

 inches in length ; and with some smaller ones of a pale brown pyroxene 

 thickly scattered through a white matrix, in which, here and there, are 

 tufts of small yellow ciystals of a mineral which probably is epidote. 

 The mass of the Makari belt and of the dyke at Caburi are of a compact 

 rock of similar composition. Its specific gravity varies from 2"67 to 

 2-78. In thin slices the rocks are seen to be made up of large plates 

 of orthoclase, smaller ones of microcline, and some of plagioclase with 

 bent lamellae ; large patches, and more or less fractured idiomorphic 

 crystals of a colourless i:)yroxene, probably an augite, with, in places, 

 streams of crystalline plates of clinozoisite with some of epidote. 

 The pyroxene crystals frequently are bordered with epidote, and 

 a little sjjhene is also jDresent. In j^laces tufts of radiating microliths 

 interpenetrate the alkali-feldspars from the patches of clinozoisite. 



A dyke of a very basic syenite of about thirty feet in breadth 

 traverses hornblende-granitite-gneiss in the Teboco channel of the 

 Mazaruni River. It is very coarse-textured ; the inner parts of the 

 dyke consist of abundant, short, broad phenocrysts of dark-coloured 

 hornblende, some being from a quarter to half an inch across lying 

 in a compact light grey matrix, whilst in the outer parts, near the 

 junction of the dyke with the gneiss, the crystals of hornblende, 

 there very abundant, are relatively long and narrow. In places near 

 its margins the rock is traversed by thin veins of epidote and zoisite. 

 Its specific gravity varies from 2"89 to 3"01, being highest near the 

 edges of the dyke. Under the microscope the rock is seen to consist of 

 idiomorphic crystals of green and brown hornblende, containing in 

 places irregularly bounded plates of a colourless augite, with some 

 irregular masses of the same mineral lying in a matrix of alkali-feldspar 

 — which in places shows the characteristics of microcline — and some 

 quartz. Near its margin the rock of the dyke contains many grains of 

 magnetite, some of which occur in the matrix, while others are included 

 in plates of hornblende. It is traversed by veins of eijidote, and here 

 and there contains small quantities of scattered granules and aggregates 

 of epidote with some zoisite. Quartz in small, irregularl}^ sloped patches, 

 which are clearly of secondary origin, freipiently occurs in the rock. 



The rock is a basic augite-hornblende-syenite. 



Mica-gabbro. — In many places in the Mazaruni River, and in parts 

 of the Pomeroon River, great masses of a very coarse-grained diabase 

 or fine-grained gabbro occur. The rock differs from the later intrusive 



