o'28 Y KIKUCHI 



ed in a «lightly reddish coloured slate looking like ' Hornfels ', and has 

 a marked tendency to separate along the basal face. The hardness is 

 somewhat low ( H = 2*5). Before the blowpipe, it fuses slightly at 

 the edge. The crystal is built up exactly in the same manner as the 

 unaltered crystals from the Watarase-gawa region, and therefore the 

 forms of the transverse and longitudinal sections are exactly alike to 

 those already described. The separation along the basal face is caused 

 by the regular arrangement of mica-flakes parallel to the Base (001) of 

 the crystal. This arrangement may be well observed in the longitud- 

 inal section. Figure 11 is a magnified view of this section, in which 

 the line lines are the cleavage traces of the mica-flakes. The black 

 dots arranged in the form of rows or wedires, are the inclusions of 

 carbonaceous matter. 



Examined under crossed Niçois, the layers of mica extinguish 

 the light simultaneously, and therefore form optically a con- 

 tinuous individual, but they are usually intermixed with irregul- 

 arly arranged aggregates of the same mineral. The green mica 

 is faintly but distinctly dichroic : when the ray of light vibrates 

 parallel to the direction of the basal cleavage traces, it is of a 

 light greenish colour, but is of a light yellowish tinge when the ray 

 vibrates at right angles to this direction. 



A. Wichmann* distinguishes two phases in the alteration of 

 Cordierite. For example in case of Cldoropliyllite, the first stage is 

 characterized by the formation along the fissures of a certain crystal- 

 line fibrous substance, while in the second stage the alteration occurs 

 essentially in the mica flakes. The two modes of alteration which we 

 have distinguished above evidently correspond to these two stages. 

 Wichmann observes that the transition of the crystalline substance 



• Die Pseudomorphosen des Cordierite — Zeitschft. d. deut. geol. Gesellft. 1874, Bd. XXVI 

 p. 079. 



