434 



MINERALOGY 



rhombic in outline. Tremolite is colorless ; actinolite, light green ; 

 hornblende, olive to brown. All varieties show the characteristic 

 prismatic cleavage of 124 30' when cut perpendicular to the ver- 

 tical axis. The relief is distinct and the surface is rough, especially 

 in the dark-colored varieties. Pleochroism is very characteristic 

 and increases with the depth of color, absorption being the stronger 

 parallel to the cleavage. Interference colors are higher, second 

 order, or very high in case of some of the basaltic forms. Extinc- 

 tion varies from to 20 with the percentage of iron present, 

 though not regularly, being higher in the darker varieties. The 

 plane of the optic axes is in the plane of symmetry with the acute 

 bisectrix in the small angle P, Fig. 347, page 199. 



Interference figures are well defined, an .optic axis emerging 

 in the basal and orthopinacoidal sections. Distinguished from 



FIG. 478. Section of Diorite, showing the Relief and Cleavage Angle, 124, of 

 Hornblende a, and also the Parallel Cleavage b, when cut Parallel to the Vertical 

 Axis. 



pyroxenes by the cleavage; by the strong pleochroism; smaller 

 angle of extinction ; lower interference colors and somewhat lower 

 relief. 



The amphiboles are important rock-forming minerals widely dis- 

 tributed in igneous and metamorphic rocks. Tremolite and ac- 



