PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



201 



Diabase.] 



be different from the pyroxene in this rock generally. The fact that extinction takes 

 place parallel to the fibrous inclusions seems to be the character which determined 

 him to designate it enstatite, since in enstatite such alteration to greenish bastite, 

 the axis of the fibres being parallel to the vertical axis of the original mineral, is a 

 common phenomenon. 



COMPARATIVE CHARACTERS OF ENSTATITE AND AUGITE. 



Enstatite. 



Orthorhombic. 



Specific gravity, 3.1. 



Easy cleavage, 010. 



Positive bisectrix, n s , 



Plane of the optic axis, 010. 



Dispersion about s p > v. 



(+) 2 V=70. 



n t (yellow)=1.674. 



it. =1.669. 



,, =1.665. 



n e n,, =0.009 (mean) weak. 



Prismatic angle of scant cleavages, in basal 



sections. 

 (110 A 1TO)=88 20 '. 



Refraction, strong. 



OOl 



100 



Section 010. 



FIG. 11. OPTICAL SCHEME 

 OF ENSTATITE. 



Talcose fibrous alteration in easy cleavage 

 parallel to 010; axis of the fibres parallel to 

 the vertical axis. 



Easy (brachypinacoidal) cleavage always par- 

 allel. 



Prismatic cleavage, imperfect and irregular. 



Section 100 shows bisectrix perpendicular. 

 Optically positive. 



Augite. 



Monoclinic. 



Specific gravity, 3.3 3.5. 

 Easy cleavage, 110, 1TO. 

 Positive bisectrix n ,; 

 Plane of the optic axes, 010. 



+ 2 V = 60 to 68. 



n,. = 1.733 to 1.728. 



H , = 1.717 to 1.712. 



it ,, = 1.712 to 1.706. 



jig n ,, = 0.022, strong. 



Prismatic angle, to which easy cleavages 

 agree, shown by basal sections. 



(110 A I'O) = 87. 



Twinned with itself on 100 and rarely on T22; 

 with amphibole on 100; with biotite, 001 of 

 the mica is applied to 110 of augite. 



Refraction, strong. 



IOC 



~OQI 



0/0 



FIG. 12. OPTICAL SCHEME 

 OP AUGITE. 



Schillerization in secondary cleavage parallel 

 to 100; its elongation is perpendicular to 

 the edge 100:010 (diallage). 



Easy (prismatic) cleavages are all parallel in 

 sections 100 and 010. Other sections in the 

 zone 001:100 have the angle of the cleavages 

 bisected by the extinction position. 



Section 100 shows an optic axis oblique. 



Optically positive. 



It is evident that the distinguishing characters are to be found in the comparison 

 of the double refraction, the specific gravity, and the direction and perfection of the 



