PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



661 



Esterellyte.] 



ortho-axis, the extinction directions are different in the two parts of the crystal. More- 

 over, in one section, cut parallel to the clinopinacoicl, parting parallel to the basal 

 plane is seen, and this runs straight through the colored rim and the colorless core. 

 From these facts it is seen that the two parts of the crystal have the same crystallo- 

 graphic axes, i. e ., are parallel growths, but that the axes of optical elasticity, excepting 

 the one coincident in direction with the ortho-axis, do not have the same directions in 

 the two parts of the crystal. 



" The green crystals and rims have a lower index of refraction, lower double 

 refraction and a smaller extinction angle than the colorless augite (the extinction 

 angle measured being that between a and c in acute angle b} . The dark green crystals 

 are more pleochroic than the lighter ones, and the colorless ones show no 

 pleochroism. These facts indicate that the green crystals and rims contain more 

 of the acmite molecule than the colorless parts. (That the augite contains a 

 considerable amount of the acmite molecule is shown by the analysis given below.) 



FIG. 4:i. SECTION OF AUGITE CRYSTAL SHOWING AREAS OF DIFFERENT OPTICAL ORIENTATION. 



" The extinction angle of the colorless augite in sections parallel to the clinopiua- 

 coid runs as low as 37, although usually higher than this; this is an angle of 53 as 

 the extinction of augite is usually measured, /. c., c to c in obtuse angle b. In the 

 green crystals and rims a is inclined about 22 to <, but in one section it is as low as 

 18. The positions of the axes of elasticity with reference to the crystal lographic 



