THE ARCHiEAX FORMATTO\ OF THE AP.ÜKÜMA PT.ATEAU. 229 



end-prodnct of decomposition of the last two rainevnls. the hornhlende 

 and hiotite, are chloritic substances, while feldspars end in ])rodncin,£r 

 a psendophitic mineral, which is regularly arranged with reference 

 to the crystallographic outlines of the original mineral. This is 

 rendered visihle in the aggregate colours of polarization by the 

 peculiar position of foliar of a rauscovite-looking mineral. 



B. Structural varieties of Granites. 



a) Amphibole -Granite. 



The prinr-ipal rock of the Ahukuma plateau, the amphibole- 

 granité, shows a great uniformity in its mineralogical composition, but 

 varies locally in texture with the relative proportions of the composing- 

 minerals. It consists essentially of (piartz, orthoclase, plagioclase, 

 microcline, hornl)lende, and biotite, the last two ingredients being 

 present nearly in equal quantities. As all granites contain some 

 quantities of micas, I suggest fn- the present rock the name horn- 

 blende-granite. The accessories are a])atite, zirc(^n, and iron ores ; 

 titanite is rare. Tlie general coloiu* is rather greyish ; but the 

 weathered rock is of a rosy tint, owing to the oxidation of iron in 

 the feldspars, and may 1)e disringuislied as the pink-granite.' 



For the type of this amphil^ole-granite may be taken that 

 from (Jomi-sawa, in the Jigoku-road, 1^ ri south-east of Ono-shin- 

 machi. It is coarse crystalline, and its crystals of horrdilende attain a 

 length of more than 1 cm. The deep oily-brown biotite is entirely or 

 partially enclosed by the am]ihibole ; consequently the former precedes 

 the latter in the act of cryst:dlization. The ])ellucid needles of 

 ayxitite acciunulate at tlie margin of these coloured minerals. Plagio- 

 clase, with a rather coarse twinning-lamellation, is idiomorphic in 



1 See ante p. 220. 



