GNEISS AND LIMESTONE CONTACT PHENOMENA 269 



The microcline is finely cross-hatched. The feldspars and the 

 quartz form the aplitic mass in which scales of muscovite, biotite, 

 and calcite are scattered. 



In the muscovite, |8d = 1.6 15 =•=0.003, 2,nd in the biotite ^0 = 

 7d = 1.650='= 0.003. Unfortunately there are not yet sufficient data 

 to establish the composition of the micas from the refractive 

 indices. The biotite probably has about 80 per cent of the iron 

 compound.^ 



The mode of this rock was calculated with the following result: 



Quartz 27.7 Biotite 4.0 



Albite 38.2 Calcite 2.0 



Anorthite 3.2 Apatite 0.2 



Microcline. 15 -o 



Muscovite 10. o i°°-3 



This is a rock in which calcite occurs as a primary mineral. 

 Emerson remarks {op. cit., p. 153): "Because of its proximity to 

 the Coles Brook limestone it contains microscopic grains of calcite." 



All the analyses of the Becket gneiss show an ordinary granitic 

 composition with about equal amounts of potash and soda and 

 very little lime. A study of thin sections of a number of specimens 

 proved this to be a general rule. Some of them, however, do not 

 contain any potash feldspar, but all are rich in mica. 



a monochromatic illuminator and Dr. Merwin's dispersion diagram for the set of 

 liquids (see E. Posnjak and H. E. Merwin, Jour. Amer. Chem. Soc, Oct., 1922). In the 

 case of feldspars, where birefringence is low and dispersion sufficiently known, this 

 method offers the advantage that all the indices of refraction can be determined with 

 only one liqiiid. In the actual work it was found convenient either to determine /3 

 only, or to seek grains showing the highest birefringence and determine, in three of 

 four grains, a and y', taking the lowest and highest values found. The composition 

 was then located on the curves published by F. E. Wright, Am. Jour. Sci. (4) (1913), 

 PP- 36, 540. The latter is the quickest way and sufficiently accurate to give the com- 

 position of the plagioclase within the limits of 2 or 3 mol. per cent. 



' The lowest indices of refraction in biotites recorded are those in yellowish brown 

 biotite from Vesuvius, 015 = 1.5412; 7d = i-574S, and the highest those in black biotite 

 from Somma: ai> = 1.5795; Td = 1-638(3). The highest value of 7 is much lower than 

 that often found during this work. The maximum value, found in a biotite from 

 biotite gneiss, railroad line between Becket and Washington, was 70=1.660=^0.003. 

 Taking this value and 7d = i-574 as limits a very rough approximate estimation of 

 the composition of biotites may be obtained graphically. 



