THE ARCHAEAN FORMATION OF THE ABUKUMA PLATEAU. 269 



the behaviour of different polarization -colours in the two-halves, as 

 well as by unequal angles of extinction on the right and left. 

 Its polysynthetic lamellar structure is rarely to be observed, and 

 then only partially, being chiefly confined to the margins of crystals, 

 and coming to an end abruptly near their middle. Maximum- 

 extinction takes place usually at very small angles with the twin- 

 ning-sutures, a fact which compels me to place this feldspar in the 

 andesine series. The great majority of feldspars occur, however, 

 in simple crystals, and should therefore be considered as orthoclase. 

 Some of the orthoclastic feldspar have acquired an " eye "-like 

 habitus, being of somewhat larger size than the neighbouring grains, 

 and encircled by the foliœ of biotite. The feldspar-" eye " has been 

 transformed into scales like muscovite, and encloses a mass of iron- 

 glance disposed in such a manner as to assume a fluidal structure. 

 Opaque iron-ijlance (not magnetite) enters into the composition of 

 the schist in such large quantity as would allow one to speak of the 

 rock as being literally charged with iron ore, and is found especially 

 in those parts where the foliar of biotite have been brought together 

 into a thick heap. Taking into account the considerable amount of 

 feldspar present in the rock, I propose to call it the feldspar - 

 biotite-schist. The rocks examined were brought from Shimo-ichino- 

 kaya (Sono-aua), Iwa-mae göri ; and Kamesaku, Ij ri east of Ota 

 in Hitachi. 



ß) Another modification, a true biotite-schist, is rather fine- 

 granular, black or greenish-black, and a perfectly fissile, evenly 

 plane-parallel schist, being made up of alternations of thin bands of 

 biotite and quartz. A typical specimen of this rock is found at 

 Gongendö (Kami-Yoshima), not far from the town of Taira. 

 Feldspars seem not to be present, and if present are rare. Tourmaline 

 is the most characteristic accessory component, and through its 



