278 PENTTI ESKOLA 



mingtonite, forming the inner parts of the aggregates of amphibole 

 (positive, 2V large, 180 = 1-650; about 40 mol. per cent FeSiOj), 

 quartz, ilmenite, apatite, zircon. 



Almandite amphibolite, band in gneiss, roadside W. of Benson 

 Pond. Plagioclase (a:'D = 1.540; 7'^ = 1.548; An^s), almandite 

 (nD = i.8o6=»=o.ooi; probable composition^ 70 mol. per cent alman- 

 dite, 15 mol. per cent pyrope and 15 mol. per cent grossularite) , 

 hornblende (/3d = 1.665; Fe4o) microcline, biotite ((8 = 7 = 1.648; 

 about 80 per cent Fe-compound) , ilmenite, apatite. 



Biotite gneiss, railroad line between Washington and Becket 

 stations. Quartz, plagioclase (An^) microcline, biotite (i3D = 7D = 

 1.660, chiefly iron compound), hornblende (010 = 1.697; j8d = 1.708; 

 7d = 1.709;^ 2Va small Fcgo), magnetite, epidote (cud = 1.741 =^ 0.002 ; 

 /8d = 1.760=^=0.005; 2V = 75° appr.; about 35 mol. per cent iron 

 comp.) . 



THE mg:fe proportion in the various bands of the gneiss 



AND IN ITS different MAFIC MINERALS 



A comparison of the Fe:Mg proportions, as determined from 

 the refractive indices of the pyroxenes, amphiboles, and biotites 

 in the various kinds of gneiss, shows at once that they are controlled 

 by definite laws. For the sake of convenience, the values of the 

 atomic portions of Fe as a percentage of Fe+Mg (briefly called 

 the Fe-quotient) are tabulated (p. 279). 



In this table, the rocks are arranged in the order of decreasing 

 Fe-quotient in their amphiboles and pyroxenes. Now, comparing 

 this order with the statements of the approximate quantitative 

 proportions of the minerals given in the preceding chapter, we find 

 at once that it is, also, at least nearly the order of increasing basicity 

 or decreasing acidity, excepting, however, the two last-named 

 examples of clinopyroxene gneiss. Their case will be discussed 

 below. 



'P. Eskola, "On the Eclogites of Norway," Videnskapsselsk. Skri. Mat.-natiirv. 

 Kl. I, No. 8 (1922), p. 9. 



^ The indices match the highest values represented in the amphiboles studied 

 by Ford (Amer.Jour. 5a., Vol. XXXVII [i9i4],p. 181), i.e., those of the hornblende 

 from Cornwall, Orange County, New York, which has a mean index 1.7 1 and con- 

 tains 23.35 per cent FeO and 7.41 per cent FeaOj against 1.90 per cent MgO and 

 12.10 per cent AI2O3. 



