ON THE SO-CALLED CKYSTALLIXE SCHISTS OF CHICHIBU. 107 



platy, more or Ics.s graphitic variety, while as we advance to the 

 higher zone, the rock pa^ises into a very thin, stiff, papery .schist. 



Beginning now with the lower étage, the thick-phity, grayish- 

 white variety easily c]eaves off into slabs of more than y, centim. in 

 thickness. The u]iper and lower surfaces of the slabs are covered by a 

 thin skin of grayish-green, soft, talcose lamellae which, when viewed 

 by reflected light, shine with a glittering lustre. It is the presence 

 of this layer Avhich allows of the rock being so easily taken off'. 



A newly fractured surface presents layers of a snowy-white, 

 saccharoidal mass, consisting of a fine admixture of quartz and 

 felspar-grains, the presence of the latter being shown by their pearly 

 lustre. The relative quantity of both minerals could not be easily 

 ascertained on account of the approximate similarity of their optical 

 behaviours. Tlie felspar shows sometimes signs of cleavage ; and it 

 as well as the quartz displays an undulatory extinction which in 

 no wise ever comes to total darkness. On the whole, the felspar 

 seems to be the predominating constituent, and usually more por- 

 phyritic in its habitus than the quartz-grains. 



Thin talcose, wrinkled lamellae were treated with a few dro['S of 

 HFl, and then evaporated to a concentrated solution, out of which, 

 on cooling, many crystals of potassium-silico-fluoride with the forms 

 belonging to the isometric system, aggregated themselves into perfect, 

 delicate shapes. The angle of the optical axis is as wide as in 

 muscovite. This sericite Js either colourless, or light-green, and has 

 fibrillated texture, but is irregularly outlined ; smooth lamellae are not 

 rare, in which case it is sometimes almost impossible to discriminate 

 the green sericite from the co-existent chlorite. 



The sericite- aggregate is never found in a pure state. In 

 the slides, sericite itself is not abundantly present, being sliced off' 



