CHAPTER VII. 



THE PETROGRAPHY OF THE QUARTZ-PORPHYRY, 

 GRANOPHYRE, PORPHYRITES, AND ALLIED ROCKS. 



These rocks are developed to a considerable extent in British Guiana, 

 and in parts occupy very broad areas. They are of at least two ages, 

 the more important variety being older than the granitic rocks whilst 

 of later date than the fundamental gneiss, the less important and 

 relatively rare kind being either offsets from the granitic rocks, or later 

 intrusions through them. The latter will be described with the 

 granitic rocks. 



The series includes both massive and schistose members, the latter 

 being derived from the former by dynamic metamorphism. 



The massive varieties are divisible into the following groups : — 



L Quartz-porphyry and quartz-porphyrite. 



2. Granophyre. 



b. Feldspar-porphyrite. 



4. Augito-porphyrite. 



5. Hornblende-porphyrite. 



6. Felsite. 



No strict lines of demarcation can be drawn between these groups, 

 as they shade almost imperceptibly from one into another. Similarly, 

 it is not possible to demarcate the massive from the schistose varieties, 

 the change from one to the other being very gradual. I have classed 

 as quartz-porphyries, or as quartz-porphyrites, rocks containing numerous 

 blebs or phenocrysts of quartz, whether accompanied or not by 

 phenocrysts of feldspar, in a microgranitic, microgranular, micro- 

 crystalline, granophyric, or felsitic groundmass ; as porphyrite those with 

 phenocrysts of plagioclase-feldspar in a microgranitic, microgranular 

 microcrj^stalline, or felsitic groundmass ; as augite-porphyrite those with 

 phenocrysts of augite ; as hornblende-porphyrite those with pheno- 

 crysts of hornblende ; as granophyre those containing phenocrysts of 

 feldspar, and a few of quartz in a micropegmatic groundmass : and as 

 felsite some rocks which do not contain phenocrysts, and which 

 usually have either a microgranitic or a microgranular structure, but in 

 places consist of an irresolvable felsitic mass. 



As a rule the plagioclase-feldspars are far more in evidence in the 

 rocks containing blebs or phenocrysts of quartz than are the orthoclase 

 ones, and hence the term quai'tz-porphyrite is generally more applicable 

 to them than is that of quartz-porphyry. 



