54 The Gi'ology of tlie Gold Fields of Jh'itixh Guiana. 



1. (a) Quartz-jjorphyri/. — The (juai-tz-porphyries are coaipact rocks, 

 varying in colour from greenish-grey to very dark grey, and ranging 

 in specific gravity from 2"57 to 2"74. Their groundmass is either 

 microcrystalHne, microgranitic, mierogranular, or micropegmatitic. In 

 the microcrystalHne varieties the interlacing, minute, lath-shaped 

 crystals of feldspar, usually plagioclastic, show more or less corroded 

 edges, and have, in varying degrees, the more or less noticeable 

 flow arrangement termed " pilotaxitic." The microgranitic, micro- 

 granular and granophyric mati'ices are composed of feldspar and 

 (juartz, the former usually being predominant. The feldspar consists 

 of orthoclase and plagioclase in varying proportions. Sericite is of 

 frequent occurrence, sometimes in considerable quantity, while minute 

 granules of epidote are found in places scattered through the mass. 



(Small rounded blebs of quartz, more or less corroded l)y the matrix, 

 and usually of uniform extinction, but in some specimens showing 

 strain shadows, and phenocrysts of feldspar — generally orthoclase, but 

 in places plagioclase — of very varying sizes, are scattered through tiie 

 groundmass. Many of the orthoclase-phenocrysts are clouded by 

 sericite, while, in places, the plagioclase ones contain numerous minute 

 granules and crystals of epidote. In some specimens small patches of 

 colourless augite, more or less changed to epidote, nests of chloritised 

 biotite, small scales and aggregates of gi'eenish biotite, and a few small 

 flakes of muscovite occur, whilst small patches of pale green hornblende 

 are spai'sely present. Minute crystals and grains of magnetite and of 

 titaniferous iron ore, granules of sphene, small prisms of ajiatite, 

 patches of carbonates, and crystals of pyrite are accessories, usually in 

 un important quantities. 



(Some (juartz-porphyries from the Berloice River district are either 

 pink, red or purple in colour, contain numei'ous small blebs of quartz 

 and dull earthy-white phenocrysts of feldspar, and have a finely 

 laminated structure. They appear to have been originally tuffs. 

 Their groundmass is crypto-cr3^stalline or felsitic, and in parts contains 

 a good deal of glass. Minute, irregularly shaped blebs of glass occur in 

 places. Some of the rocks are more or less silicified and are traversed 

 by narrow veins of (juartz. 



The principal interest in the rocks of the Berbice River lies in the 

 quartz-porphyry and allied rocks. They differ from rocks of the same 

 group collected in the auriferous districts of the colony by showing 

 fewer signs of dynamic metamorphism, and by retaining in some cases 

 their original hyalopilitic or pilotaxitic structures sufficiently to enable 

 an opinion to be formed as to their origin. They have been either 

 rhyolites and andesites, or altered and consolidated tuft's derived from 

 similar rocks. 



Quarfz-porplryrites. — The quartz-porphyries grade almost imper- 

 ceptibly into (luartz-porjihyrites, the latter differing from the former by 

 the smaller proportions of orthoclase -feldspar present in them, whilst 

 the feldspar-phenocrysts consist only of plagioclase. 



