230 SMITH— PARAGENESIS OF MINERALS. [Octobers, 



Calaveras Valley, is a massive quartz diorite, in which the meta- 

 morphism has only just begun, and all the original minerals may be 

 recognized. The rock is composed of original quartz, oligoclase, 

 and brown hornblende (probably kataphorite), with a little secon- 

 dary lawsonite and glaucophane (crossite). The readjustment con- 

 sists in the partial breaking up of some of the oligoclase, and the 

 addition of some of the albite molecule to the borders of the brown 

 hornblende to form a fringe of glaucophane (crossite). Since the 

 glaucophane contains no titanium, this constituent crystallized out 

 around the borders of the parental mineral as titanite and leucoxene. 

 Most of the oligoclases are still perfectly fresh, and those partially 

 decomposed still show their outlines and twinning bands, giving 

 evidence of decomposition in cloudiness due to kaolinization. The 

 little lime they contained went to formation of lawsonite, w^hich is 

 present in minute crystals, with the characteristic optical properties 

 of that mineral. 



The original oligoclase, from its extinction angles on the twinning 

 bands, has the composition of AbgAn^, with 63 per cent, silica. The 

 small quantity of the anorthite molecule set free simply took up water 

 and formed lawsonite, CaAlgSisOg + 2H2O. The albite molecule 

 was disintegrated, giving sodium silicate to the hornblende to form 

 glaucophane, and setting free kaolin and silica ; 2NaAlSi308 + 

 2H0O = NaoSiO., + sSiOo + H^Al.Si.Og + H.O (kaolin) . In this 

 none of the sodium silicate and little of the silica was left free, 

 but went into combination with the hornblende on the borders to 

 raise the percentage of silica and soda to that necessary to form 

 glaucophane. The rock is composed of approximately four parts 

 oligoclase, two of hornblende and one of quartz. This has nearly 

 the same composition as that of the albite-crossite gneiss of Berkeley, 

 and the albite-glaucophane gneiss of Angel Island. The analysis 

 of the quartz diorite with secondary crossite and lawsonite is given 

 below. 



About three miles west of Redwood is a massive gneissic rock 

 composed of approximately one-fourth lawsonite, one-third glauco- 

 phane, one-third silica (quartz), and the remaining one-twelfth of 

 garnet, sphene and white mica. Every mineral enumerated above 

 is clearly secondary, and yet the analysis given below is that of a 



