232 SMITH— PARAGENESIS OF MINERALS. [Octobers, 



expense of any mineral now visible in the rock. The original rock 

 must have been richer in magnesia than the needs of the glauco- 

 phane demanded, and as no garnets or epidotes were formed the 

 excess of magnesium silicate crystallized out as talc, in this case a 

 primary constituent of the rock, and not a weathering product. This 

 could only happen where the metamorphism was not very intense, and 

 the absence of garnets would show that this was the case. No 

 analyses have been made of this rock, but an estimate made from 

 a study of numerous slides shows that the original material must 

 have been a basic igneous rock or a basic tuff. 



Sedimentary Rocks. — The altered sediments containing glauco- 

 phane are : ( i ) Basic tuffs, probably of the character of diabase ; 

 (2) arkose, of the character of granites or diorites ; (3) sandstones, 

 with varying proportions of impure clay ; (4) clay shales, with vary- 

 ing proportions of sand and other impurities. 



The basic tuffs consist largely of augitic and feldspathic frag- 

 ments, and these recrystallize very readily. From this material have 

 been formed schists composed largely of glaucophane and epidote, 

 with accessory titanite, garnet and zoisite. The reactions here are 

 the same as in the recrystallization of diabases and gabbros, the high 

 percentage of iron usually necessitating the formation of abundant 

 epidote. 



While feldspars are usually present in small quantities in the 

 altered acid sediments, they are known in large quantities in but few 

 places in California. The arkoses usually have the composition of 

 granites or acid diorites, and the recrystallization products are of 

 the same chemical nature. In the gneiss from Melitta near Santa 

 Rosa the original rock was an arkose composed of fragmental horn- 

 blende, orthoclase, acid plagioclase, and quartz. The orthoclase has - 

 recrystallized as such, the original hornblende has formed actinolite, 

 and some of it has taken up soda, forming mixed crystals of carin- 

 thine and glaucophane. Some of the original ferro-magnesian min- 

 eral was probably titaniferous, for titanite is abundant in the rock. 

 The excess of iron, alumina and magnesia from the partial decom- 

 position of the hornblende and plagioclase formed abundant red 

 garnets. It is noteworthy that neither epidote nor lawsonite nor any 

 pyroxene was formed. 



