The Fetrograjihy of tha Quartz-porphyry, eU\ 55 



2. Granoj)hyrp. — lu many places, especially in the vicinity of the 

 larger intrusive masses of diabase, quartz-porj^hyries and porphyrites 

 having a micropegmatitic groundmass occur. They are compact rocks 

 with usually inconspicuous blebs of quartz, and many phenocrysts of 

 feldspar. In colour they vary from light grey to greyish-green and, 

 as a rule, are of the lighter shade. Their specific gravities range from 

 2'6-J to 2-72. The rocks are made up of a groundmass of a micro- 

 pegmatite of quartz and feldspar, the latter in places being ortho- 

 clase, in others albite, in which are embedded small blebs of quartz and 

 abundant phenocrysts of plagioclase, and occasionally of orthoclase. 

 In places the micropegmatite is replaced by granitic or granular 

 aggregates of (^uartz and feldspar. The phenocrysts and the accessory 

 minerals are similar to those described as present in the quartz- 

 porphyries and porphy rites. 



Fi'ld!<par-porphyriti'!<. — These rocks differ from the foregoing by not 

 containing blebs or phenocrysts of quartz and the feldspars are generally 

 plagioclastic. Phenocrysts of plagioclase, and, in a few instances, of 

 orthoclase, are present, the former usually in abundance. The rocks 

 i-ange in colour from greenish-grey and bluish-grey to dai^k -green, dark- 

 grey and purplish-grey, and in specific gravity from 2'68 to 2'82. 

 Their groundmass is either microgranular or microgranitic, and consists 

 of feldspar with some ijuartz. Lying in the groundmass are relatively 

 large phenocrysts of plagioclase-feldspar, frequently clouded with 

 sericite, zoisite and epidote. Small nests and patches of green biotite, 

 of chlorite, and of epidote occur scattered in the rock. The accessory 

 minerals present are the same as those found in the quartz-porphyries 

 and quartz-porphyrites. 



In some of the feldspar-porphy rites the groundmass is made up of 

 ill-defined laths of feldspar, which in places passes to a feldspar- 

 micromosaic. These are greenish-grey rocks of specific gravity from 

 2'70 to 2*82. In addition to phenocrysts of feldspar they contain a 

 few small ones of a colourless augite, or of a pale hornblende. 

 Chlorite, viridite, sericite and epidote are frequently present in the 

 groundmass. 



In the lower parts of the Potaro River, — but, as far as ray experience 

 goes, rarely elsewhere, — feldspar-porphyrite occurs having for ground- 

 mass minute interlacing lath-shaped crystals of feldspar, many of which 

 are plagioclase, their edges being more or less corroded. They have a 

 pilotaxitic arrangement, the feldspar-laths lying in streams with their 

 longer axes approximately parallel to one another. These rocks contain 

 suiall phenocrysts of feldspar, but not in any abundance. Their accessory 

 minerals are the same as those found in the porphyries, but, as a rule, 

 they are present in very small proportions, or are almost entirely 

 absent, whilst the ferromagnesian minerals seldom, if ever, occur. The 

 rocks are essentially feldspathic ones closely allied to Bostonite. 



Some specimens from the Cuyuni and Berbice Rivers are grey and 

 purplish-grey rocks of specific gravity 2-75 to 2-78. The groundmass 

 of these is distinctly andesitic and tlow-structure is fairly well marked 



