504 



NEW YORK STATE MUSEUM 



Grenville rocks. The quartzose rocks are the only members of 

 the Grenville series sufficiently well shown within the quadrangle 

 to repay careful study. In general their mineralogy is compara- 

 tively simple, as they are made up of quartz, feldspar, pyroxene 

 and phlogopite mica in varying amounts. Small amounts of various 

 accessory minerals are also present but seldom in sufficient quantity 

 to affect the general statement. In general there is little difficulty 

 in distinguishing the quartz and feldspar under the microscope, 

 and mostly the feldspar admits of fairly accurate microscopic de- 

 termination. So far as- they are concerned therefore, the rocks 

 admit of reasonably accurate microscopic analysis. But the pyrox- 

 ene and mica are minerals of very varying composition, and as a 

 first step it was desirable to have analyses of them available, for 

 use in calculating the results of the microscopic rock analyses. For 

 a pyroxene analysis a rock was selected free from mica, and with 

 only accessory titanite and zircon present in addition to the quartz, 

 feldspar and pyroxene. The titanite was fairly abundant and oc- 

 curred included in all the three constituent minerals, so that it was 

 impossible to wholly separate it from the pyroxene, but with this 

 exception an absolutely clean pyroxene powder was obtained with 

 Thoulet solution. Since titanites do not vary widely from their 

 theoretical composition, it seemed that, by using the amount of 

 TiOa obtained in the analysis as a base for calculating out the 

 titanite, the remainder must closely represent the composition of 

 the pyroxene. 



Analysis of pyroxene from quartz-pyroxene gneiss (3-C-4) i mile east 

 of Grampus lake 



Analysis 



Titanite 



Pyroxene 



To 100% 



SiO^.. 



A1203 



Fe,03 

 FeO . 

 MgO . 

 CaO. . 

 NasO 

 K2O. 

 Mn02 

 H2O. 

 TiO^. 



52.87 

 3 -oi 

 1.76 

 6.21 



17.68 



14-47 

 0.98 

 o .29 

 0.32 

 o . 10 



43 



1.36 



99-57 



1.88 



4.67 



51-44 

 3.01 

 1 .76 

 6.21 

 17.68 

 13 . 1 1 

 0.98 

 o . 29 

 0.32 



54.20 



3-17 

 1.86 



6.54 

 18.63 

 13.81 

 1 .03 

 0.31 

 o. 34 



94.90 



I E. W. Morley, analyst. 



