86 B. KOTÖ. 



is of an indigo-blue colour, and shows flat prismatic forms ; all ar- 

 ranged parallel to each other, and also parallel to one prevailing hori- 

 zontal direction in regard to the rock itself Thus a thin slide of the 

 rock in any one direction shows only the same side of glaucophane 

 contained in it. 



I, Basal Section. 



Any one of the vicinal basal sections in a thin slide presents usual- 

 ly an approximate hexagonal outline, being bounded by four long, and 

 two short sides, {fig. II.) These sides are the traces of go P (110), 

 and CO P <x) (010) ; those of the orthopinacoid never happened to be 

 observed in any slide. The prismatic angle is about 124°, similar to 

 that of a hornblende. The cleavage- traces upon x P (110) are distinct 

 and well developed, but all are widely separated. (/^. 17. é fig. V.) 

 Only in a few cases the cleavage-direction parallel to the clinopinacoid 

 ( GO P c^ ) is seen, although less clear, {fig. V.) On rotating the section 

 cut nearly at ± to the vertical axis, over the polarizer, the crystals are 

 seen to change from a light brown to a light bluish purple; and ex- 

 tinguish the light in the direction of both diagonals. We often 

 observed the crystals attaching by the right prismatic face of the one 

 with that of the left of another individual, or even one individual 

 penetrates the other, {fig. II.) In the third case then, three or more 

 crystals, are packed together by the prismatic faces so as to form a 

 definite aggregate. Again, we have still another instance in which 

 the glaucophane shows an intergrowth of two individuals, placed in 

 such a manner that the orthopinacoid (go P ôo ) is the contact plane. 

 On account of their minute size, here the character of interference ring 

 could not be made out. Whether all these cases stated above are the 

 outcomes from a mere intergrowth of crystal-individuals, or in some 

 way connected with the twining formation, I am at present not able 

 to say, as these are not capable of a rigid proof. 



