130 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Other hand, zoisite has been abundant!}' proved by Hunt, Cathrein, and 

 others to be peculiarly characteristic of rocks which, whether regarded as 

 decomposed eruptives, as metamorphic, or as original crystalline sediments, 

 indicate the formation of this mineral at moderate temperatures. 



Taking all these circumstances into consideration, it appears that 

 scarcely a single mineral species could have been selected which would 

 afford a better criterion of the prevalence, at the epoch of its formation, 

 of temperatures decidedly below a red heat, and probably much below. 

 Zoisite might conceivably occur in two ways, and it is not improbable that 

 it actuall}' does so occur. If found among mere decomposition-products, 

 replacing primary minerals pseudomorphically as aggregates, it would seem 

 to prove that the rock in which it occurred had simply been decomposed 

 under physical conditions appropriate to the formation of zoisite But, if 

 zoisite is found embedded in clear, continuous masses of minerals which 

 can be shown to be authigenetic, it would seem to afford conclusive evi- 

 dence that these minerals have been formed at moderate temperatures. This 

 latter mode is characteristic of a great portion of the rocks of the Coast 

 Ranges and very often in cases where from the mere inspection of slides 

 it might readily be supposed that the material under examination was 

 eru])tive. 



While zoisite appears to form an admirable indication of the metamor- 

 phic character of the holocrystalline rocks of California, it must not be sup- 

 posed that their determination as altered sediments is dependent on the 

 identification of zoisite. Before the mineral character of the constituent 

 which proved to be zoisite vi'as known, there was abundant evidence, both 

 from field examination and from microscopical study, that the rocks in 

 question were metamorphic, and the conclusions would remain substantially 

 as here presented if it should be ])roved that zoisite is a common, original 

 constituent of the most typical eruptive diabases and diorites. The present 

 investigation may properly be regarded as proving, quite independer.tly of 

 its chemical constitution or of its occurrence in whatever class of rocks else- 

 where, the wide distribution in metamorphic rocks of zoisite both as an 

 authigenetic constituent and as one of the first of these constituents in the 

 order of development. 



