82 



Vol. XLIIL, Art. G. - S. Tsuboi : 



Most calcic. 

 More calcic. 

 Less calcic. 

 Ijeast calcic. 



Fig. 36. 



this mineral often form a grouped phenocryst. Twinning is usually 

 present in simple and polysynthetic lamellae according to the albite, 

 Carlsbad, and rarely pericline laws. Zonal structure due to chem- 

 ical différence is often very faintly exhibited. The mode of 

 zoning is not always very simple, the more calcic and less calcic 



feldspars being often alternately 

 developed in successive zones 

 (Fig. 36). It is a general rule, 

 however, that the outermost one 

 coating each crystal in a very 

 thin shell is the least calcic and the innermost one is the most 

 calcic. The refractive indices measured with the isolated piec33 

 are: «<l,572</5<1.578<r, so the mineral was identified as calcic 

 bytownite with its chemical composition Ab,,Ans7 according to 

 Becke.'^ The optic axial angle was determined to be approximately 

 88^ supposing ,5=1.57.5. Tiie mineral contains numerous inclusions 

 (^f augite, feldspar, glass, and fluid bubbles. These are often 

 arranged in distinct zones orientating themselves parallel to the 

 outline of the host. 



The plagioclase in the groundmass is too fine to be determined 

 accurately, but as the mineral has its refractive indices near 1.57 

 in average it may be slightly less calcic (sodic byfcownite) than 

 the phenocrystic one. It is prismoid in crystal habit and is very 

 commonly twinned in two or three lamella?. 



Another mode of occurrence of plagioclase which deserves 

 special mention is as " rhombic lamellae " (Pi. 111. Fig. 2). These 

 are found imbedded in brown glass and are often so extremely 

 thin that their action on polarized light is scarcely recognizable. 



1) " Zur PhysiograpLie der (îemen;;teile der krystallinen Schiefer," Denkschriften der kui- 

 .'ipsli hin Akademie der Wissoisdftfle ■, 73 Band, S. 103, 1!)13 (Wien). 



