1 86 



University of California. 



[Vol. i. 



shown in Plate 1 1 , Fig. 2. More rarely they are united on a face 

 oi the prism, as in Fig. 2. In basal sections the prismatic cleavage 

 is very clearly marked, giving the rhomb-shaped network so char- 

 acteristic for amphibole. Twinning has not been observed. 



Figure 2. — Showing the forms of cross sections and grouping of prisms of 

 the blue amphibole. In c the center of the prism is occupied by albite. 



Optical Characters. — The optical investigation of the amphibole is 

 rendered difficult by the intensity of the pleochroism in all sections. 

 The plane of symmetry is the plane of the optic axes, and an axis 

 emerges very excentrically in orthopinacoidal sections. The posi- 

 tion of the acute bisectrix, and, consequently, the optical sign of 

 the mineral, was not determined. Repeated measurements of the 

 extinction in sections cut parallel, or approximately parallel, to the 

 clinopinacoicl were made with the Bertrand ocular, and the maxi- 

 mum values thus obtained were as follows: — 



I3°3. 13°, I2 °5> I2 °- 2 - I2 °- 1 1 °-5 ; Il0 » I o°-9- 

 We may thus consider the direction of extinction as making an 

 angle of about 13 with the chief crystallographic axis. The axis 

 of elasticity corresponding to this direction of extinction was deter- 

 mined by means of the quarter-undulation mica plate as the axis of 

 maximum elasticity, tt. This orientation of the elasticity axes 

 appears to be the most important optical character for the deter- 

 mination of this mineral. It places it in close relation to riebeckite, 

 which alone of all the amphiboles possesses this optical orientation. 

 Although the observation could not be made in this mineral, it 



