ON THE SO-CALLED CRYSTALLINE SCHISTS OF CHICHIBU. 



89 



fibrous lamellae of sericite. The same structure also occurs in all the 

 rocks belonging to the Sanibagawan series, and may be considered as 

 characteristic of these rocks. The same is also typically developed 

 in the graphite-sericitc schist, to which we shall have occasion to 

 refer afterwards. 



Quartz-grains are frequently found as enclosures in the felspars 

 without anv definite crvstalloirranhic relation to the latter. In 

 massive rocks, felspars belong to an earlier generation in crystallizing 

 out from the rock-magma, but here a portion of quartz has solidified 

 prior to that of alknli-alumina silicites. so that the physical condition 

 durinfy the formation of the rock must have been a ditt'erent one.^) 



The sericite is of a yellowish-white, or light grass-green colour 

 w^ith silky lustre. In the latter case, it is pleochrijic, showing 

 strong absorptions parallel to the axis of a plication. In some 

 places as in Azuhata in the province of Hitachi, a rock similar to the 

 ' pierre ollaire ' or Giltstein of the Alps is found, being essentinlly 

 made up of sericite with a few admixtures of quartz. This affords 

 us a good material for the stud}^ of sericite. It is a light greenish- 

 white mineral with a perfectly smooth surface of cleavnge and is 

 extremely thin-lamellar. Its stauroscopic figure shows a tolerably 

 wide axial-angle like that of muscovite. The sericite was isolated l)y 

 means of the Thoulet solution from a glaucophane-bearing rock 

 occurring in Otakisan near Tokusima in Awa, and Mr. Tnkayama, 

 of the Geological Survey of Nippon, kindly undertook a chemical 

 analvsis of the mineral, the result of which is as follows : — 



SiO, 53.01 



AUG, 34.70 



1) Michel- L''vy calls those round quartz-graias irregularly distributed in. orthoclase 

 " Quartz do corrosion," and considers them to be of a secondary origin. J. Koth, All- 

 gemeine u. Chemische Geologie, II Band, III Heft, p. 393. 1887. 



