The Petrogrdphy of the Qunrfz-pnrphyry, pIc. 59 



quartz, and in the darker varieties with more or less chlorite and 

 some granules of epidote. Sericite is always present in the 

 more compact forms in patches and films scattered through the 

 mass of the rock, and in the fissile ones in streams and veins 

 marking the foliation. In a few places the rock is traversed by 

 streams" of minute flakes of greenish-biotite. Small grains of 

 magnetite or of titaniferous iron-ore with leucoxene, of sphene, 

 of epidote, and occasionally cubes of pyrite are present. 

 Phenocrysts of plagioclase commonly occur, and are frequently 

 fractured or more or less distorted. Where not fi-aclured they 

 usually show strain-shadows, and in places exhibit signs of 

 commencing granulation ; many of the phenocrysts are clouded 

 with zoisite^ and epidote and to a great extent with sericite. 

 The rocks in places contain much calcite and limonite as 

 decomposition products. 

 ((•) Chlorite and CMoritoid-schists.—Angite and hornblenrle- 

 porphyrites in an unaltered condition are of somewhat rare 

 occurrence in the colony, but chlorite-schists and other rocks 

 presumably derived from rocks of intermediate composition are of 

 somewhat common occurrence intercalated with sericite-schists. 



The chlorite-schists are usually green, greyish-green or dark- 

 green in colour, and they vary in specific gravity from 2-70 ti> 2-87. 

 They generally have a more or less well-marked silky feel, 

 and "are fissile in very varying degree. They are made up of a 

 micro-mosaic of feldspar and some quartz, with actinolitic horn- 

 blende, usuallj^ colourless, much green chlorite, some sericite, 

 and a few granules of epidote. In the more fissile varieties the 

 mass of rock is traversed by numerous streams of chlorite, and 

 in parts by films and narrow veins of quartz. In some places 

 phenocrysts of plagioclase, generally much altered, are 

 indistinctly seen. In a few specimens plates and patches of 

 pale-green or pale-blue hornblende are present, whilst in others 

 the remains of phenocrysts of augite or of hornblende are 

 represented by more or less angular patches of chloritv-, actinolitic 

 hornblende, and epidote. Minute grains of magnetite, of 

 titaniferous iron-ore and of pyrite form unimportant accessories. 

 Plates and patches of calcite are present in many specimens. 



Near the junction of the Potaro and Esseipiibo Rivers some 

 greyish-green compact rocks, varying in specific gravity from 2-73 

 to 2-82, are found. They have a soapy feel, like that of serpentine, 

 and they differ from the usual class of schistose rocks of the 

 colony by containing chloritoid or ottrelite. Where the leaves 

 of chloritoid traverse the mass in streams the rock assumes a 

 schistose structure. The groundmass of the rock is microgranular, 

 and is made up of quartz with a little feldspar and a good deal of 

 nearly ist)tropic viridite ; some small grains of magnetite and of 

 titaniferous iron-ore are scattered through it, and here and there 

 patches and aggregates of epidote are found in it. Leaves of 

 chloritoid are plentifully scattered through the rock. 



