OX PYROXENE COMPONENTS TN" CERTAIN VOLCANIC ROCKS. 



SM 



other. On closer examination however, it is found that the 

 Auo-ite shows a stronger interference-colour. Also n certain section 

 shows an oblique angle of extinction making 40° with the direction of 

 the vertical axis, such being a clinopinacoidal section generally formed 

 of the traces of the faces ocPcô , -P, o P. 



The section at right angles to the vertical axis shows a trace 

 of cleavage parallel to ooP. Sometimes it is fourni that this section 

 formed of ooPöö , ce Poo, and zc\\ is separated into two halves by 

 a trace of a twin-boundary parallel to ccPdc . As we cannot in 

 o-eneral have the section filling exactlv at riirht angles to the vertical 

 axis, such a section generally shows a difference in the angular values 

 of extinction, with respect to the line of boundary. This is in fact 

 a common form of the Augite-twin, with ooPco as a twinning face, 

 which cannot of course be expected in the Rhombic system. In 

 the ball-like masses of the glassy rock obtained from Ototojima this 

 kind of Augite is specially well observed in association with t he 

 Rhombic Pyroxene, with Plagioclase both as well-defined crystals and 

 as 'Pthombic lamellae,' and with some Pieotite. 



More usually, however, the Augite is developed in the glassy 

 rocks of Chichijima as very slender crystals, sometimes almost 

 tilling up the entire glass-mass in confused aggregates so as to 

 make the glass resinous in lustre. The development of these crystals, 

 as indicated on pag. 70. it always rudimentary, having the 

 tendency to assume fern-like shapes somewhat simulating the Pitch- 

 si one of Arran in Scotland. The crystal always appears in elong- 

 ated form along the vertical axis, the angle of extinction in 

 clinopinacoidal section measuring nearly 40°. The cross section 

 is always six sided formed by the prism-faces, and a pinacoid ; the 

 former presenting a peculiarly concave outline* (fig. 13c). The 



*On account of the small size, and the curved character of the faces, the exact orientation 

 of this section cannot be determined. 



