276 



PENTTI ESKOLA 



The prominent characters of the mineral composition of the 

 various kind of gneiss are stated in the following. The mineral 

 constituents of each rock are named in the approximate order of 

 abundance. The refractive indices were determined, if not stated 

 otherwise, with a maximum error of =1=0.003 i^^ biotite and horn- 

 blende and epidote, and =1= 0.002 in plagioclase and pyroxene. 

 The composition of the minerals is given in terms of mol. per cent, 

 stating: in plagioclases the percentage of anorthite (An), in 

 clinopyroxenes (diopside-hedenbergite) the percentage of heden- 

 bergite (He), in hornblende the total Fe as a percentage of Fe+Mg,^ 

 and in the biotites and epidotes (very roughly) the percentages 

 of their iron compounds. 



Aplitic clinopyroxene gneiss around the inclusions of skarn. 

 Mainly microcline, subordinate plagioclase (Ang), quartz, clino- 

 pyroxene (/3d = 1.705, 7d = 1.727, 11642), epidote, variable, sometimes 

 very poor in Fe,^ calcite. 



Banded clinopyroxene gneiss (Fig. 3) in biotite gneiss, W. of 

 Benson Pond. Microcline, quartz, plagioclase (q;'d< 1.536; 7^0 = 

 1.544; Auis), clinopyroxene (Heeg),^ titanite, epidote. 



I Calculated from the diagrams given by W. E. Ford, Amer. Jour. Sci., 

 Vol. XXXVII (1914), P- 185. 



- 1 measured in one case for different wave-lengths, with a maximum error 

 =•=0.001: 



These values are lower than any given in the literature (cf. M. Goldschlag, loc. cit.). 

 This mineral is, however, monoclinic, and not a zoisite. 2V = 90° appr. 



3 In it were determined, with a maximum error =1=0.001: 



The birefringence suggested that this clinopyroxene is richer in hedenbergite 

 than any other found in this tract. Its refractive indices for various wave-lengths 

 are in fair agreement with those of DijiHee, as shown on the diagram. Its extinction 



