THE ARCH^AN FORMATION OF THE ABUKUMA PLATEAU. £49 



The feldspars are of tAvo kinds, the «jiie ortlioclastic, the other 

 plagioclastic ; the Jatter is minute in size, poJysynthetically twinn- 

 ed with the angle of extinction of 8*'-4° with the sutures of the fine 

 lamellae. The orthoclase exhibits dull-yellowish polai'ization-colours, 

 and encloses often the twinned feldspar. Boihfeldspars are reiiiarJca- 

 hhi on account of tlicir enclosures of iiianij cßiartz -grains with diferent 

 optical orientations. 



The micas are also of two kinds ; the tomhac-ljrown biotite in 

 ragged lamellae without ;iny crystallographic outline, with unusually 

 dark ])leochr()ic halos, including rutile-needles in the sagenitic form ; 

 the muscovite only in slight quantity, enclosing and often enclosed 

 by the biotite. Accessories iwe zircon, apatite, :ind iron-glance. 



The rock ihmi Mshiyama, 1^ ri s<3uth from Takanuki, contains 

 many stiff, disjointed needles of sillimanite (Fig. 9, PI. XX III) ; the 

 one from the Nakayama pass, on the Nakayaina road, from Tanagura 

 to the province of Hitachi, contains a greeii. [il)rous biotite together 

 with white mica-. This rook is specialkj interesting in the possession of 

 impressions like etclied fgures on the surface of the cnjstals of its feldspars^ 

 whicli are disposed one after another In series, parallel t<> the princpial axis 

 of the crystal. Moreover, large crystals of ide;dly pure andalusite (3 

 mm.) are present irregular in outline, and encircled by confused ao-- 

 gregates of fibres of muscovite. A few of the crystals are coloured, 

 and then are pleochroic, and rose-red when the ])rincipal nxis of the 

 crystal is par:dlel with the short diagonal of the polarizer. I'here are 

 some traces of prismatic cleavage, which are made more distinct by 

 the infiltration of the hydrous sesquioxide of iron. 



A h) Two- Mica- Schist. 



A typical specimen of this variety may be found in the cliff near 

 Sôri, not fu' from Ishikawa. On a cursory glance it appears just like 



