56 The Geoloijy of the Gold Fields of British Guiana. 



in them. They diflfer from the rocks of the Bostonite type by containing 

 small phenocrysts of augite and Hakes of more or less chloritised 



hiotite. 



4 and 5. Augite- and Hornblende-porphyrites . — Porphyrites having 

 well-marked phenocrysts of augite and hornblende are of far less common 

 occurrence than are the quartz and f eld spar-porphy rites. Some of them 

 resemble consolidated tufi's, and pi'obal:ily have originated from volcanic 

 debris. 



4. Auyite-porphyrite. — This is a dark -grey rock of specific gravity 

 from 2"85 to 2'86. As a rule its groundmass is a confused micro- 

 crystalline to microgranular mosaic of feldspar with chlorite, epidote, 

 zoisite, sericite, titaniferous iron-ore with leucoxene in minute grains, 

 and some secondary (|uartz. Colourless hornblende in minute laths and 

 needles and a few Hakes of greenish biotite are found in places. Very 

 pale to colourless augite is present in them in small phenocrysts, which 

 in places are more or less fractured, whilst some of them have actinolitic 

 boiderings. Some phenocrysts of plagioclase also occur in the augite- 

 porphy rites, 



5. Hornhlende-'porpliyrite. — The groundmass of the hornblende- 

 bearing porphyrites is very similar to that of the augite-porphyrites, 

 but, as a rule, needles of colourless hornblende are more abundant in it 

 than in that of the latter. Some specimens are traversed by films of 

 secondary biotite. The hornblende-phenocrysts vary considerably in 

 size in different specimens. The hornblende is very pale-green to very 

 pale-blue in colour, and in some specimens occurs in laths and in patches 

 as well as in phenociysts. In some places the hornblende is more or 

 less changed to epidote, whilst here and there the phenocrysts are 

 bordered with numerous minute grains of magnetite extruded from 

 them. The rocks contain small phenocrysts of plagioclase-feldspar, 

 usually clouded by inclusions of sericite, zoisite, and epidote. The 

 hornblende-porphyrite is compact in structure, greenish-grey to dark- 

 grey in colour, antl in specific gravity it varies from 2"77 to 2'89. 



6. Felsite. — This forms very compact rocks of various shades of grey 

 to almost black, and of green, red, chocolate-coloured, dark brown, and 

 deep purple. It varies in specific gravity from 2"62 to 2-78. It is 

 made up of either a crypto-crystalline mass, containing in places 

 granules of glass, or of a ver}^ finely grained micro-mosaic of feldspar 

 with sericite and some quartz. In the darker-coloured varieties, chlorite 

 and widely difiused minute grains of magnetite are present in some 

 quantities. In the red and chocolate coloured sorts hematite is found 

 in a very finely divided state. Films and thin veins of (juartz in places 

 traverse the rock, whilst some specimens are more or less silicified l)y 

 secondary quartz. As secondary constituents, carbonates in plates and 

 in patches, and pyrite in small cubical crystals, are often present. 



