comp()ni<:nt minerals. 77 



tion is 6'>> &>• a. Tlie angle of extinction is that of anipliibole, hut the 

 interference colors are of lower onh-r. The specific gravity is 3.10 io 3.11,' 

 but the mineral is usually so intimately associated with others as to make 

 a perfect separation difficult. 



The genetic relations of the glaucophane are not entirely clear. In 

 the greater number of cases it is closely associated with ordinary actinolite, 

 and there appear to be luiquestionable transitions bet^Yeen the two. Thus, 

 one portion of an area of entirely undecomposed amjohibole of uniform 

 orientation is often bi-ight blue, another green, and these pronounced tints 

 shade off into each other by imperceptible gradations. Had only these oc- 

 cun-enccs been observed, the conclusion would have been almost inevita- 

 ble that the two varieties of amphibole had been produced simultaneously 

 and by the same methods. There are other cases, however, in whicli nar- 

 row streaks of the blue mineral appear along the junction of actinolite 

 crystals, which suggest the possibility of epigenesis of glaucophane upon 

 actinolite. I am inclined to consider this suggestion misleading, however, 

 because fibration and sensible difference of orientation would almost inev- 

 itably result from such a process. 



zoisitc. — ^hich the most interesting mineral yet detected in the rocks 

 undergoing metasomatic recrystallization is zoisite, which as an important 

 I'ock-forming mineral has hitherto been observed only in the saussuritic 

 crystalline schists and gabbros. In the rocks of the Coast Ranges this 

 mineral is one of the first indications of recrystallization; it is found in 

 slides of every group of the recrystallized rocks and is often jiresent in 

 large quantities, especially in the schists. 



The zoisite presents no good cleavage, but traces of fissility parallel 

 to the main axis are sometimes visible. The prisms are usually jointed and 

 terminal faces are often distinct. Measurements of the projection of the 

 interfacial angle between the brachydome and brachypinacoid agree with 

 the real value of this angle as Avell as could be expected. Square cross-sec- 

 tions are nut luiconimon and often only a single corner appears to be trun- 

 cated. This irregular development of faces in the vertical zone is charac- 

 teristic of zoisite.^ 



' LUdccko found the .specific gravity of glaiicojiliane fmni Svra, 3.101 (Kiitli : AUr. mid chem. Geol., 

 p. 21). 



- Daiia'.i System i.f :\Iiiieialo-y, )i. 20n, 



