386 O. E. LE ROY — GEOLOGY OF RIGAUD MOUNTAIN, CANADA 



in small longitudinal plates arranged along the cleavage planes. A few 

 grains of pyrite of cubic outline, with an alteration border of hematite, 

 completes the list of constituents. 



An analysis of the freshest rock procurable was made, the results of 

 which are placed in the left-hand column, and, for the sake of comparison, 

 one of the Plauen sj^enite* is given in the right. 



I. II. 



SiO., 62.62 59.83 



A1263 15.69 16.85 



FeA 2.07 — 



FeO 4.73 7.01 



MnO 10 — 



CaO 2.60 4.43 



MgO..... 1.61 2.61 



Na,0 3.87 2.44 



K,0 5.95 6.57 



H2O 61 1.29 



99.76 101.03 



Specific gravity 2.68 2.73 



I. Hornblende syenite, Rigaud (analyzed by O. E. Le Roy). 

 II. Hornblende syenite, Plauen bei Dresden * (analyst unknown). 



The analysis of the Rigaud syenite bears out the result of the micro- 

 scopic examination in that, as shown by the ratio of the alkalies, the 

 orthoclase is the predominant feldspar in the microperthite. The rock 

 is a normal sj^enite and compares favorably in composition with the 

 typical rock of Plauen. The latter is higher in lime, iron, and magnesia 

 than the former, and lower in silica, which is to be accounted for by its 

 containing more of the ferro-magnesian constituent and less quartz. 

 The total alkalies are almost the same for both rocks. 



QUARTZ-SYENITE PORPHYRY 



The porphyry, as has been })reviously mentioned, differentiates from 

 a quartz syenite variety in the west of the mass to a quartz porphyry in 

 the east, with an intermediate transitional band between. 



Megascopically, the quartz-syenite porphyry consists of phenocrysts 

 of feldspar, wdth a few of quartz, imbedded in a dark gray felsitic ground- 

 mass, which has an irregular conchoidal fracture. The feldspar, when 

 fresh, is brownish gray in color, with good cleavage and high luster. It 

 Aveathers to a pale red or grayish white. The phenocrysts have a poor 

 form and vary considerably in size, from almost microscopic forms to 



* Rosenbusch : Gesteinelehrbuch, chapter on Syenites. 



