COMPOSITION OF HYPERSTHENE-DIABASE. 343 



Analyses of Feldspar. 



1. 11. Ratio of I. 



SiO, 51.40 51.03 .856 .856 



Al„6, 30.98 1 Qi ir r.304-( 



FeA -22 )^^'^^ i.OOll 



MnO trace trace 



■^"= -"" ""■"" } 31-15 {:,",« I -305 



trace 



CuO 13.40 13.92 .240 | ,..^ 



MgO .45 .59 .010} -^^^ 



K2O .39 .39 .004 I ^.^ 



INa.^O 2.85 2.85 .046 j '"^^ 



99.69 99.93 



E2O : RO : K2O3 : Si02 = 1:5:6:17, which gives 5 (Ca2Al4Si40i6) '• 2 (Na^ Al^SigO J 

 or 5 An : 2 Ab. 



The analyses show this part of the feldspar to be labradorite. The optical 

 properties mentioned above point to the presence of anorthite. We judge, 

 therefore, that there are at least two varieties of feldspar present. 



The monoclinic pyroxene is nearly black when the grains of it are seen in 

 reflected light. In thin sections the grains always have an irregular outline. 

 Very feeble diochroism from greenish gray to greenish yellow can be detected. 

 In most sections the interference colors are brilliant. Crystals cut parallel 

 to the clinopinacoid show an angle of extinction slightly greater than 40°. 

 Sections in which the perfect prismatic cleavage is at right angles give an 

 axis in converging light. Such sections also show cleavage parallel to the 

 clinopinacoid and interpositions and cleavage parallel to the orthopinacoid. 

 This latter cleavage is the principal, if not the only, w^ay of distinguishing 

 between diallage and augite in thin sections. The resemblance to the dial- 

 lagic augite, described by E. Boricky as occurring in the melaphyres from 

 Bohemia,* is in some particulars quite striking. The interpositions are so 

 numerous as to give the augite a fibrous appearance in ordinary transmitted 

 light. They can be resolved with a high power into fine needles lying in 

 planes parallel to the base, with their long direction parallel to the ortho- 

 diagonal. Hence, in sections from the orthodiagonal zone, they appear as 

 parallel lines of acicular microliths, while in clinopinacoidal sections they 

 appear as points, arranged in lines making an angle of about 74° with the 

 prismatic and pinacoidal cleavages. 



Simple and polysynthetic twinning parallel to the orthopinacoid is quite 

 common. Clinopinacoidal sections of such twins show the lines of interpo- 

 sitions meeting each other at an angle of about 150°. In polarized light, 

 lamellae parallel with these lines indicate polysynthetic twinning parallel to 

 the base. 



The diallagic augite is the first mineral in this rock to undergo decompo- 

 sition. 



* Rutley ; The Study of Rocks, 1880, p. 125. 



