76 Y. KIKUCHI 



v. Rath (Amblystegite) 



e : e (212 : 2Ï2) = 153° 11' 152° 22 %' 



e : e (212 : 212) = 12.1° 12' 121° 8 1 /,' 



i : in (211 : 110) = 138° 35' 138° 55' 



a: m (100 : 110)=- 136° 5' 135° 50' 



From the last value the prism-angle of the Pyroxene would become 

 92° 10'. 



The characteristic cleavage of the prism is distinctly developed, 

 the cleaved face exhibiting a pearly lustre. Cleavage parallel to 

 Brachypinacoid is scarcely observable, but it can sometimes be recog- 

 nised as distinct fissures in microscopic slides cut at right angles to 

 the Vertical axis. The crystal has the tendency to separate along 

 the Macropinacoid a, the plane of separation presenting an uneven 

 surface. This is however not due to any interposition, as is usual in 

 the massive variety of this mineral. 



The phenomenon of pleochroism is very marked. The axial 

 colours examined along the three pinacoidal faces gave with a section 

 of nearly *5 mm. ; a = reddish-brown, ß = greenish-yellow, y = 

 yellowish-green, with a weak absorption a > ß >> y. These characters 

 agree with the observations of Hatch* on the Rhombic Pyroxene of 

 Charchani. In still thinner sections the pleochroism becomes very faint. 

 The section parallel to the Base, when examined under a conver- 

 gent polarised ray, shows the emergence of the positive bisectrix y 

 the plane of the optic axes lying parallel to Brachypinacoid b, while 

 in the section parallel to the Macropinacoid a, the negative bisectrix 

 a emerges perpendicularly. 



The characteristics given above chiefly refer to the larger crystals 

 which are found scattered within the glassy rocks, but the microscopic 



* Vulkangruppe von Arequipa— Tscherrnak's Min. u. Petro. Mitth. Bel. 7. 1887. p. 339. 



