NOTES ON SOME IGNEOUS ROCKS OF JAPAN 



569 



marked by irregular cracks, rilled with isotropic, low refractive, 

 and colorless, substance. Twinning according to the albite law 

 is common. 



Olivine occurs in two-sized crystals. The larger ones are abun- 

 dant and play an important role as microscopic phenocrysts. Their 

 shapes are equant or prismoid. In many instances, the outlines 

 of crystals are irregular by invasion of groundmass, sometimes 



Fig. 3. — Andesine phenocryst, natural size 



extremely narrow and deep, parallel to crystallographic faces, 

 showing the successive growth of the mineral, but the general out- 

 lines are referable to crystal forms (Figs. 5 and 6). Though 

 distinctly cracked they are entirely fresh and inclose clouded 

 glass, but are free from other inclusions. 



Augite forms magnophyric crystals which are very rarely seen 

 in hand specimens. The minute grains in the base, showing high 

 refraction, appear to be augite. 



