SOME OCCURRENCES OF PIEDMONTITE IN JAPAN. 307 



usually obtain sections approximately parallel to the 6-axis; but there 

 is a mai'ked 'difference in the colours of various sections so as to lead 

 observers to think of entirely other minerals, the one being a deep 

 violet, the other a brownish-yellow. As there are great differences 

 in the axial colours already stated, it may be naturally expected that 

 a section parallel to the basal pinacoid M is of a brownish-yellow (the 

 flicial colour of c + b); and that which is taken nearly parallel to the 

 orthopinacoid T of a deep violet (the facial colour of a+t>). The 

 clinopinacoidal section shows the deepest shade of colour, the facial 

 colour being of a combination of c and a. 



The extinction-direction is, of course, parallel and at right angles 

 to the longer sides of sections in the zone of 31 and T, and the 

 intensity of colours also depends upon the section in this zone. The 

 polarization colours are magnificent, varying from an intense violet to 

 an indigo-blae tinge, which become more pronounced, if we insert a 

 Quartz-plate in the tube of the microscope. 



The Piedmontite is ideally pure; neither liquid- or gas-inclosures 

 nor any microlithic interpositions are discernible. The mineral was 

 isolated from the other constituents of the schist obtained from Otaki- 

 san, Awa province, by means of the Thoulet solution ; and the 

 chemical analysis was kindly undertaken by Mr. J. Takayama, of the 

 Geological Survey of Japan, with the following result : — 



SiO, 36,16 



AI2O3 22,52 



Fe, O3 9,33 



Mil, O3 6,43 



Ca 22,05 



MgO 0,40 



K, trace 



Na^O ,44 



H,0 3,20 



100,53 

 H : Ca :: 1 : 2.2 Ca : it : Si : : 1.25 : 1 : 1.92 



