R. A. Daly. 



0° 10° 20° SO 



40° 



Amphiboles and Pyroxenes. 

 50° 60° 70° 80° 90° 



2 V= 00 



2 F = 80 



2 r= 70' 



ihlj j!i 



2 v=m'- 



2 r= 50 



Plate I. — Showing the curves of extinction in the vertical zone of Amphiboles 

 for which the extinction on (010) is 10° and the optical angle is 50°, 60°, 70°, 80°, or 

 90°. The abscissa represents tlie amount of rotation of a plane in the zone away 

 from the plane of symmetry. The ordinate is the corresponding angle of extinc- 

 tion for that position of the movable plane. The position and amount of maximum 

 extinction are represented by arrow-heads. Tiie values of extinction for 10° inter- 

 vals in the zone are entered in the diagram. 



