5o6 



NEW YORK STATE MUSEUM 



anorthite was assumed to form Ab 2 An x with the albite. The result 

 is given in the table, which agrees closely with the microscopic 

 analysis, though the silica is a little higher, and the alumina cor- 



Chemical composition of quartz- pyroxene gneiss, calculated from 

 mode and partial analysis 





Qz. 



Orth. 



Alb. 



Anor. 



Pyr. 



Tit. 



Zir. 



Total 



Si0 2 



A1 2 3 



56.69 



1.47 

 b .42 



9-47 

 2 .69 



2.96 9.43 



0.78 



O. 20 



80.90 



Fe 2 3 







O.32 

 I. 14 



3 25 



2.41 

 0. r8 

 . 05 

 .06 







0.32 



FeO 















MgO 















3-25 

 4.48 

 1 .81 



CaO 









1.38 



.69 





Na 2 







1.63 



K 2 





o-39 







0.44 



Mn0 2 











Ti0 2 











0.99 



O . 42 



0.99 

 . 42 



ZrO, 



























Total 



56.69 



2.28 



13-79 



6.87 



17-39 



2 . 46 



. 62 100 . OO 



respondingly lower than in that. The more probable cause for the 

 difference is a failure in all cases to distinguish between quartz and 

 feldspar in the slide. But the totals would be but slightly affected, 

 and the character of the rock as surely indicated in the one case as 

 in the other. 



The analysis shows a distinctively sedimentary rock, a metamor- 

 phosed sandstone which was somewhat shaly, and somewhat cal- 

 careous. The low alumina, comparatively high lime and magnesia 

 and low alkalis show a wide discrepancy when compared with any 

 igneous rock of similar silica percentage. There is however close 

 agreement with the composition of such a sandstone as indicated. 

 While such origin had been inferred from the appearance of the 

 rock in the field, and in thin section, it seems to be put beyond 

 reasonable doubt by the analysis. 



In the purer varieties of these rocks, such as are found for example 

 on the hill in the extreme southeast corner of the quadrangle, where 

 they are exposed in large thickness, the rock is entirely composed of 

 quartz and white pyroxene, with here and there a little bit of 

 graphite. The rock is foliated and the thin section is certainly richer 

 in pyroxene than the main rock, and therefore not adapted to 

 analysis. But the mineralogic make-up is such that there is no 

 question as to the character of the rock. Its silica percentage would 

 be somewhat higher than in the previous case, not far from 85%, 

 the remainder being mostly lime and magnesia. 



