26 KOTO : NOTES ON THE GEOLOGY 



UYPEESTHENE. 



Hypersthene takes the place of olivine in some Basalts of 

 the Pescadores ; consequently the presence of one totally excludes 

 that of the other, — a state of thing quite exceptional to the modern 

 Japanese Andésite of a glassy, black, porphyritic type, in whicli 

 both minerals api^ear always concomitantly. We have then the 

 Hypersthene-Basalt, in lieu of the Basalt proper. It is a note- 

 worthy fact that this stray variety of rock seems to be wide- 

 spread, at least in my specimens, in the out of the way islets, 

 such as Irapai, Kin-sho, and Hatto, the only exception being 

 the one from Sei-kei (West Valley) in Hoko, though I could 

 not find a sufficient reason accounting for the special dis- 

 tribution of this hypersthene-bearing rock. 



It is usually a comparatively easy task to discriminate 

 hypersthene from olivine, but in the present case some difficulty 

 is experienced in making out for certain the presence of the 

 former. 



In regard to the form, the (1) hypersthene is extremely 

 slender, being about six times longer than broad, and, as being 

 of the intratelluric origin, it lias a marginal zone deeply corroded 

 and partly granulated, and has indefinite faces at the poles 

 of the crystals (PI. II, Fig. 3). I observed once a morphotropic 

 growth of a highly-polarising, mouoclinic pyroxene around a 

 hypersthene, just as is the case in Andésites. Cleavage is 

 developed along the longest extension of the crystals. In a patch 

 of a coarse aggregate which appears as an endogeneous or homoge- 

 neous enclosure in the finer general mass, the (2) hypersthene 

 comes together with plagioclase and augite, and in this case the 

 hypersthene occurs in broad plates [PL II, Fig. 4), with only a few 



