NOTES ON SOME IGNEOUS ROCKS OF JAPAN 



563 



The olivine is entirely fresh and remarkably but irregularly 

 cracked. 



Augite is very faint yellowish or nearly colorless, and is more 

 abundant than olivine, though it is rarely present as microscopic 

 phenocrysts. As phenocrysts, it is anhedral, but in the groundmass 

 it is well shaped; elongated prisms are common. In rare instances, 

 twinning parallel to the orthopinacoid may be seen in the larger 

 crystals. In the reaction-border about the phenocrystic quartz, 

 augite is the only mineral constituent and is imbedded in brown 

 glass. Inclusions of mag- 

 netite, apatite, and glass 

 are sparingly present. 



Plagioclase is basic lab- 

 radorite and appears in 

 well - form ed , long pris- 

 moids with polysynthetic 

 twinning according to the 

 Carlsbad and albite law. 

 Zoning is almost absent. 

 Minute grains of pyroxene 

 and magnetite are present 

 as inclusions in small 

 quantity, with also a few 

 of glass. 



Quartz occurs as a por- 

 phyritic constituent, and 

 the average diameter is about 2 mm. The outline of the mineral in 

 thin section is usually irregular, but sometimes shows the bipy- 

 ramidal form referable to crystallographic faces, as seen in the 

 microphotograph (Fig. 2). Each grain of quartz is fringed with 

 a reaction-border, consisting of elongated prism and grains of 

 augite imbedded in brown glass. The minute prismoids are 

 arranged quite regularly. They are grouped radially, each group 

 containing a few crystals that converge toward the outer side of 

 the border, as seen in Fig. 2. In triangular, interstitial spaces 

 between each radial group granular augites are scattered irregu- 

 larly. In some instances, the deep invasion of the brown glass, 



Fig. 

 border. 



-Bipyramidal quartz with reaction 



X23. 



