6 University of California Publications in Geology. [Vol.7 



phate and also slowly by ferric sulphate and presumably by 

 manganic sulphate, so its solubility may have been due to a 

 combination of these salts. 



The three zones probably represent the action of long stand- 

 ing solutions on the silver minerals of each zone and what silver 

 was carried below was in the form of silver sulphate and not as 

 haloids. The bromine and iodine were presumably carried down 

 as alkali salts. The oxidation of the deposit may have been long 

 continuous or at successive intervals, but it seems to the writer 

 that a basin must have formerly existed above the deposit in 

 which the salts accumulated, and that a body of water from this 

 basin penetrated to the ore below carrying these salts. 



In the passage of the solutions downward pools were doubtless 

 formed in the ore body, which gave rise to rich pockets of the 

 haloids. The occurrence of some of the iodyrite crystals sug- 

 gests also that they were formed by fumes of free iodine acting 

 on the silver. Doubtless free iodine was formed by the decom- 

 position of the iodide or iodate in the concentrated hot solutions, 

 and it must have formed if free acid was present. 



The hydrous secondary minerals. — The secondary minerals 

 derived by the alteration of the metallic and basic constituents 

 of the deposits are characteristically present in the form of coat- 

 ings on the walls of cavities and fissures. The coatings are 

 either in very thin, minutely crystalline, drusy layers or they 

 form minute botryoidal groupings. Most of them are brilliantly 

 lustrous but the crystals are generally immeasurable. 



The hydrous manganese oxide is present in soft velvety layers 

 of impure material and in hard mammillary shells of psilomelane. 

 Much of it may have been derived from the alteration of man- 

 ganese tungstate which appears to be one of the older minerals 

 of the vein. Manganese hydroxides carrying the alkalies un- 

 doubtedly played an important part in the formation and crystal- 

 lization of the silver haloids and in its association with these it 

 was, in general, deposited first and forms the layers upon which 

 the crystals rest. 



The soluble phosphate, arsenate, and sulphate occur in the 

 lower or iodyrite zone of oxidation. They have necessarily been 

 crystallized from more concentrated solutions, while at the same 

 time they antedate the formation of much of the iodyrite. 



