324 Y. KIKUCHI 



right angles to each other, as indicated by the two .sets of bundles. 

 The planes of boundary of these two sets would be formed, in the 

 better formed crystals, by particles of the carbonaceous matter 

 existing in the mass of the slate. In the skeletal crystal such as 

 noted above, these two sets are represented by scattered bundles 

 separated by the larger interstitional spaces of the foreign material. 

 It will be observed that the growth of the outer portion also took 

 place laterally, in three directions in the position of the twin, so that 

 t lie space between each individual was also filled up with coaly par- 

 ticles. But in the inner portion this was somewhat different ; here 

 the direction of growth was at right angles to the directive influence 

 which would bring about the twin structure, and the peculiar man- 

 ner in which this inner portion is built up, in contrast to the outer, 

 must, we think, lie in some way connected with this antagonistic 

 state of the two directive forces. The arrangement of the enclosures 

 already described (p. 322) also tells us the two directions of growth. 



Pleochroism. — To the naked eye the fresh specimens appear 

 colourless, there being practically no pleochroism. But on separating 

 the fragments of the mineral from the matrix and on washing the 

 adhering impurities of iron hydroxydes, it was found they distinctly 

 exhibit a slight blueish tinge, which is generally so characteristic of 

 this mineral. The property of pleochroism, however, seems to be 

 absent in some Cordierite minerals, e.g., that described by P. Groth* 

 from Brazil. 



Corrosion-figures. — The basal section was selected for experiment- 

 ing on the production of corrosion-figures. On etching the smoothly 

 polished face with hydrofluoric acid, it was found that the different 

 sectors were covered with innumerable figures. The form of these 

 figures was nearly spindle-shaped ; and their longer axes arranged 



* Ueber farblosen Cordierit aus Brasilien— Zeitschft, f. Krystall., 1883. Bd. VII. p. 594. 



