310 ^- koto: cause of the great 



the same till we come to Kiré, whence it changes very slightly to the 

 north-west. The youngest and c(jnsequently the northernmost of 

 the series is the hornstone of the upper Neo, where it ahuts upon 

 the (jranite-porphyry of Hakusan (Hakusan Mayeyama). Near 

 the point of contact an intrusive rock of the character of I'.onthlciule- 

 porphijnj is pressed up through the stratified complex. 



The petrographical characteristics of the porphyries are here 

 given : — 



«) Granite-porphyry, — This is a rock wLich presents peculiarities in many 

 respects, and is of wide distribution throughout Japan, especially in the liighlands 

 of Hida. It was once called granite, then diorite, but now goes generally by 

 the name of porphyrite in all the Geological Survey maps, It is, however, 

 not a typical mica-hornblende-porphyrite, since it is of granitic structure, and con- 

 tains moreover a large quantity of monocliiiic feldspar. On the other hand, the 

 name diorite does not suit it. It resembles a biotite-hornblende granite most 

 closely both in mineralogical composition and in structure ; but the presence of por- 

 phyritic crystals precludes its being classified as that. It is a greyish-white, mediums 

 granular rock of dioritic aspect, decomposing into angular fnxgments, like chips of 

 a liornstone-mass. Phenocrysts are scarcely distinguishable in hand-specimens, a- 

 the component minerals are nearly of the same size. Feldsjna- and honMende enter 

 largely into its composition, and, when examined under the microscope, quartz and 

 biutite are observed, together with »uKjnetite., epidotc, zircuit, titanite, and (qmlile as 

 accessories, the last one being especially characteristic of this rock. 



The feldspars are the dominant ingredients, and fall into plagioclastic and mono- 

 clinic varieties. Which of the feldspars is present in larger tj[uantity is not easy to 

 say. iSpeaking generally, the porphyritic, tolerably idiomorphic crystals are mostly 

 but not exclusively plagioclase, with the well-developed zoning which is the charac- 

 teristic of rocks of a porphyritic structure. The feldspars are mostly oligoclase, 

 sometimes schillerized, extinguishing light at about 18° with the twinning sutures, 

 and with their centres frequently kaoliuized over a definitely marked area with 

 patches of chlorite, the surrounding zones being apparently untouched by weather- 

 ing. The feldspars are the only components that deserve the name of phenocryst. 



The rest of the mass is a granitic aggregate of feldspars mostly orthoclase. 



