268 



PENTTI ESKOLA 



areas the dark constituent is in whole or part magnetite in small octahedra. 

 .... At its contact with the graphitic rocks the rock is commonly graphitic, 

 and against the limestone it contams in many places secondary calcite grains 

 and tremolite or actinolite. 



The following chemical analyses of the Becket gneiss and a 

 related gneiss have been published:^ 



I. Gneissoid granite, Alderman's quarry, Becket, Toscanose-lassenose. 

 Analysis by G. Steiger. Specimen in Petrographic Reference Collection of 

 U.S.G.S. No. 1649. 



II. Granite gneiss, Hoosac Mountain. Toscanose. Analysis by E. T. 

 Allen. Specimen P.R.C. 1718. Collected by J. E. Wolff. Consists of quartz, 

 microcline, albite, muscovite, biotite, magnetite, titanite, epidote, apatite, 

 zircon. 



III. Composite sample of Becket gneiss from 33 localities in the Shefi&eld 

 quadrangle. Collected by Joseph BarreU and analyzed by R. C. Wells. 



Through the courtesy of Dr. E. S. Larsen of the United States 

 Geological Survey, I had the opportunity of studying the original 

 specimen of Analysis I, the Becket gneiss from Alderman's quarry. 

 It is a light gray granite, rich in muscovite. All its minerals are 

 clear and unaltered. 



In the plagioclase were determined j3d = 1.5345=*= o-ooi 5 a'n = 

 1.532=^0.002; 7'd = 1.540=^0.002,^ corresponding to Abpz. 



I F. W. Clarke, U.S. Geol. Surv., Bull. 591, pp. 33-34- 



^ The composition of the plagioclases, in this study, was ascertained mainly from 

 the indices of refraction determined in rock powder by the immersion method, using 



