The Petrography of the Quarts-porjihyry, etc. 



65 



Schistose Eock:^ 



The massive rocks may be classified according to their ultimate 

 maarmatic compositions as follows : — 



Quartz -porphyry 

 GranophjTe 

 Quartz-porphjTite 



Feldspar-porphji-ite, Cuyuni Eiver ... 

 ,, ,, Essequibo River 



Augite-porphyrite 



Homblende-porphyrite, Barama Eiver 

 ,, ,, Mazaruni Eiver 



Alaskose. 



Dacose. 



Sitkose. 



Lassenose. 



Yarangose. 



Dacose. 



Placerose. 



Tonalose. 



By comparison of the compositions of the two classes of the schists 

 in the Essequibo-Potaro district, with the compositions of the quartz- 

 porphyrite, of the gi'anoioh3're, and of the f eld spar-porphy rite from the 

 same district, it is clear that the metamorphic changes have resulted 

 in the decomposition of the feldspar, so as to liberate quartz or silica, 

 and possibly some alumina, much of the latter remaining in the altered 

 rock in the form of kaolin, or as diaspore, the minute flakes of which 

 are confused under the microscope with sericite — which resulted from 

 the alteration of the orthoclase — the removal of a good deal of the lime 

 of the feldspars as calcite, and the change of the pyroxenes and other 

 ferro-magnesian minerals into hornl)lende and chlorite, this being accom- 

 panied by the extrusion of magnetite. In places the metamorphism 

 results in the production of quartz-schists and of so-called jasper. 



In the schistose rock it is noticeable that the metamoi-phism, while 

 converting the feldspar of the groundmass into sericite or other 

 hydrous micas and into quartz, has produced little effect upon the 

 phenocrysts of quartz, few only of which are fractured or show faint 

 strain-shadows. Similarly, the feldspar-phenocrysts have suffered less 

 alteration than has the feldspar of the groundmass. 



F 



