352 G. F. Becker — Metamorphic JRocks of California. 



Though not among- the most readily recognizable minerals, it 

 can be distinguished with tolerable ease by its physical and 

 optical properties. Some of its important characteristics are as 

 follows: rather strong refraction, tint under the microscope 

 colorless to faint yellowish green, in the latter case slightly 

 pleochroitic, colors of interference bluish gray to a dull yellow, 

 this tint being sometimes pale and sometimes strong but rarely 

 or never bright. It occurs in granular aggregates and in 

 prisms; the latter are usually jointed, occasionally more than 

 twenty times. The prisms are generally fluted in the direction 

 of the main axis. Cross-sections are square or show a single 

 corner truncated. The axes of elasticity are as nearly as can 

 be observed strictly parallel to the main crystallographic axis 

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

 large and their plane is parallel to one of the pinacoidal faces. 

 Glaucophane was first recognized in rocks from California, so 

 far as I know, by M. Michel-Levy in 1878.* It appears to be 

 connected with ordinary hornblende by transitions. No special 

 comments are requisite on any of the other minerals excepting, 

 serpentine, which will be discussed below. 



All the more important processes of metasomatic recrystal- 

 lization can be traced in the altered sandstones, rocks the 

 clastic origin of which could not be doubted for a moment. 

 In many cases one of the first stages in the process is the reso- 

 lution of the clastic grains into crystalline aggregates from 

 which new minerals are again built up. Augite and horn- 

 blende have been observed forming in this manner. The 

 newly-formed ferro-magnesian silicate crystals are often com- 

 posed of groups of microlites not quite united and yet possessing 

 a common crystallographic outline. The sharp edges of these 

 minerals, their structure and the relation to the still traceable 

 clastic fragments, all forbid the supposition that they are them- 

 selves transported fragmental material. Feldspars form in the 

 same way, and in some cases it can be shown that a single 

 newly-formed feldspar with sharp crystallographic outlines 

 occupies the position of several of the original clastic grains. 

 The feldspars also crystallize along tiny veins in the slides. A 

 frequent occurrence is the resolution of quartz grains into 

 plagioclase microlites. The reaction begins on the surface of 

 the quartz grains and produces a fringe of twinned feldspar 

 microlites in positions approximately normal to the surface of 

 the residual kernel. The microlites do not merely abut against 

 the kernel, but penetrate it for a sensible distance like closely 

 set pins in a cushion. The newly-formed feldspars in the 

 altered sandstones are in part elongated microlites, in other 

 cases they are granular. Zoisite is present in nearly all the 



* fourth Annual Report State Mineralogist of California, p. 182. 



