124 ^'o'- XLIII., Art. C — S. Tsiiboi: 



andesine with' zonal structure ; the interstitial matter is acidic 

 micrographie material, and the augite is of an unusual type. If 

 this augite is interpreted to be one rich in lower-melting component, 

 then the pétrographie characters of the micro-diorite are just those 

 which might be expected to be formed by the above mentioned 

 process from the residual liquid of the magma of Oshima when 

 cooling at a moderate rate. 



From what has been said, the genesis of the micro-diorite 

 may be as follows : — 



The magma of Oshima cooled at a moderate rate in some 

 portion of the reservoir and proceeded to crystallize following the 

 ■course mentioned above, and consolidated into a mass with pétro- 

 graphie characters as seen in micro-diorite. Tiie block now under 

 discussion is a fragmented piece of this mass, wliicli was ejected 

 at the time of eruption. 



By tlie above, the development of the general features of the 

 micro-diorite can be accounted for ; but to explain some of the 

 more minute features, further assumptions are necessary. 



That the zoning of plagioclase is not always simple in that 

 calcic and sodic plagioclases often occur in alternate zones (p. 

 -99) may be ascribed to a temporary rise of temperature due to 

 the heat evolved by crystallization in the course of cooling of the 

 magma. 



As already stated, the absence of zonal structure in the augite 

 •crystals in contrast to the noticeable one in the plagioclase crystals 

 may be due to the former outstripping the latter in tlieir sinking 

 down. In spite of this process the augite crystals are not very 

 much inferior to the plagioclase crystals either in size or in amount. 

 This may be explained by supposing that the augite crystals grew 

 more rapidly than the plagioclase crystals. This supposition is 



