392 O. K. LE ROY — GEOLOGY OT^ RIGAUD MOUNTAIN, CANADA 



which case the latter forms the interior zone. Both minerals alter to a 

 3^ellow serpentinous material. 



The quartz occurs in rounded and angular grains, filling in interstices 

 between the feldspar individuals. Fluid inclusions are numerous, and 

 strain shadows invariable. 



No anah'sis of the rock is available, but from the microscopical char- 

 acteristics it seems to compare very closely with the Rigaud syenite. 



QUAR TZ-S YENITE PORPH YR Y 



The porphyry in hand specimens has a dark gray or reddish chert-like 

 base, which holds a large number of phenocr3^sts of red feldspar. The 

 rock joints with extreme ease into polygonal forms. 



Under the microscope the following minerals were disclosed : Un- 

 striated feldspar, quartz, and a few ragged plates of partially chloritized 

 hornblende in an extremely fine grained groundmass, which, with the 

 high power, resolved itself into a mosaic of quartz grains, and lath-shaped 

 individuals of untwinned feldspar, the latter often being arranged in 

 radiate clusters. The feldspar phenocrysts occur in rounded idiomorphic 

 and irregular forms, and are apparently homogeneous, but very turbid- 

 Twinning is not ver}^ common, but examples of both Baveno and Carlsbad 

 twins are present in nearly every slide. The quartz phenocrysts are few 

 in number, rounded in form, and hold numerous inclusions. 



The usual resorption phenomena are pronounced both in the feldspar 

 and the quartz, but only the latter shows secondary growth, the crenu- 

 lated border being much finer than that found in the quartz of the 

 Rigaud porphyry. 



An analysis of the groundmass of the Grenville porph3^ry was pub- 

 lished in the Geology of Canada, 1863 Report, and appears in the first 

 column below. 



I. II. 



SiO., 72.20 69.48 



AI263 12.50 14.69 



FejOs — 3.89 



FeO 3.70 1.47 



MnO — .12 



CaO 90 .90 



MgO — .68 



NajO 5.30 5.32 



K.,0 3.88 4.34 



H2O 60 .72 



99.08 101.11 



Specific gravity 2.62 2.63 



I. Qiiartz-syetiite porphyry, Grenville (analyst unknown). 

 II. Quartz-syenite porphyry, Rigaud (analyzed by O. E. Le Roy) 



