78 



liUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



The color of the zoisite, as seen by the naked eye, varies from gray to 

 deep green. In the former case it is of course colorless in thin section, and 

 this is usually the case in the augitic and hornbleudic rocks, though a f;iint 

 greenish yellow may sometimes be observed. In the glaucophane rocks the 

 color is usually deeper, and the pleochroism is then distinct in thin section, 

 c being yellowish green to light grass green and a and 6 almost colorless. 

 The absorption is hardly perceptible. The pleochroism increases with the 

 thickness of the section. The axes of elasticity, when their position can be 

 determined, are always strictly parallel to the vertical crystallographic axis 

 and to the pinacoidal faces. The angle of the optical axes is large and the 

 plane of the axes is parallel to one of the pinacoidal faces. The colors of 

 interference usually range between a bluish gray and a pale yellow, but are 

 occasionally more vivid. The intensity of the colors often seems to vary 

 considerably with the state of aggregation. 



Zoisite occurs in the phthanites as well as in the other metamorphic 

 rocks, but usually in much longer needles than in the other metamorphic 

 rocks. Fig. 1 shows both types of crystals, between which there are plenty 

 of intermediate forms. 



Fig. 1. Zui;tite niicroliles, o, !<, and <■, from a gUaucophane rock.Ko. 31, SiilpUur Ba'.k. a and h are magnified 175 diameters; 

 r, ICG diameters : rf is from minnte qnartz veins in phthanite (Xo. 51. Mt, Diablo) and is magnified 185 diameters. 



For the pur[)ose of checking the microscopical determination of this 

 mineral, two separations and analyses were made. Though great care 

 was taken in the sejiaration and purification, the character of the rocks 

 showed that only ai)[)roxiinate results were lo be expected. Ko. 98, Sul- 



