36 KOTO : NOTES ON THE GHOLOGY 



lava-flow is the only effusive that can be met with on that 

 island. 



The olivine is the sole i')henocryü\ it is variable in size (the 

 largest one measures even ö mm.), irregular in distribution, and 

 multifarious in form, some having partial crystallograpliic faces, 

 while others have none of them. The idding^itiz ition is pecu- 

 liarly inherent in the olivine of this roek-group, and I refer the 

 readers for further details to the topic : " component-minerals " 

 p. 18 et seq. By the way, I have only to mention that the name 

 iddingsite may conveniently be applied to a special transitional 

 form of the alteration of an olivine which, after passing this 

 stage, changes into dirty-green spherulitic fibres of an optically 

 positive character. 



In the felspar-rich rocks {PL /, Fir/, (j), which are prevalent 

 in the group under question, the j^lccffioclases are all approximately 

 of the same size, and surpass the augite both in dimension and 

 quantity; while in the augite-rich rocks {PL I, jig- 4), the 

 ])lagioclases are of two generations, and the larger ones behave 

 porphyritically towards the minor ones. They are lath-shaped, 

 and muUii)le-t\vinned, the terminations being imperfect aiid 

 sheafy, and these laths are thrown together in an orderless 

 plexus, which eminently characterises the structure of normal 

 Basalt in contradistinction to that of Andésite. 



The avrjite is all of a single generation, consequently uni- 

 form, but inferior in size to the phigioclase and olivine. Some 

 are rudely idiomorphic, but by fir the most of it is granu- 

 lar, occurring in groups, and filling the angular spaces left 

 by the laths of phigioclase. The augite is, as usual, of a violet- 

 brown colour, but in the specimen from Tai-san, it is almost 

 colourless in sections. It is free from foreign inclusions, and 



