52 KOTO : NOTES ON THE GEOLOGY 



ritic structure, and they look more like a druse than like a mass 

 of crystals, having a mutual relation, characteristic of plutonic 

 rocks. Thus our augiie is remarkable in many respects. 



The ground-mass consists mainly of the idiomorphic plagio- 

 clase, long-rectangular or square in shape, and of various sizes, with 

 some degree of parallel disposition. The square sections of the 

 microliths show occasionally truncation of corners by domal 

 face and at other times slightly diverge from recta ngularity on 

 the edge 001:010. The traces of cleavage run parallel to the 

 same edge, and the sutures of twins run vertically. Symmetrical 

 extinction takes place at o0°-32° with reference to the same 

 trace, showing that the plagioclase stands just at the middle of 

 the series between the sodium and the calcium labradorite^^ 

 According to Becker, these square sections, which are prismatic 

 sections in vertical positions, are very convenient for the deter- 

 mination of the microlithic plagioclase. Intermixed with the 

 felspar, we find the less idiomorphic crystals of pale augite, 

 together with rounded magnetite and the crystals of apatite. The 

 cuneiform space left by minerals being filled with the brown glass, 

 densely charged with transparent augite. The structure of the 

 rock is therefore that kind which toe call the ' orthophyric.^ In 

 the ß variety, minute felspar-needles make the greater part of 

 the ground-mass, exhibiting the typical pilotaxitic structure. 



a ÄPOANDESITES. 



{PL III, Figs. 1 and 2.) 



One variety is whitish, bleached and compact, the other is 

 green through the presence of a chloritic mineral, having a por- 



1) Becker, Amer. jour. ScL, May, 1898. 



