COMPARISON WITH (jriCKSILVKK DIOPOSITS. 239 



iron-pyrites with smaller ([uantities of co])])er, antimony, arsenic, and 

 nickel ; gold is very commonly present. 



Regarding the rocks adjoining the deposits, Mr. Becker says''^ they — 



" have in many cn.^es been greatly nioditied. Metamorphic rocks often appear to 

 have l)een converted into or replaced by more or less dolomitic carbonates by tlie 

 action of solutions. . . . Both serpentine and the nietanH)r[)hic rocks seem to 

 be subject to this conversion." 



Containing a similar association of metals, similar gangue and similar 

 altered country-rock, it seems justifiable to express the conviction that 

 similar mineral solutions have circulated in both classes of de[)osits ; and, 

 in fact, the still abundant thermal waters found in intimate connection 

 with the ore deposits in the quicksilver region closely correspond in 

 character to the inferred composition of the once existing hot springs of 

 the gold-belt. They all show free carbonic acid, as well as abundant 

 carbonates (sodic, calcic and magnesic) ; silica is always present ; usually 

 also sulphuretted hydrogen or alkaline sulphides. 



Carl>onatization of the wall-rocks of fissure veins has neither been de- 

 scribed by A. v. Groddeck nor by F. v. Sandberger in their researches, 

 though the former has found abundant sericite in many. The wall-rocks 

 of the Comstock lode, according to Mr. Becker, are rich in iron-pyrites, 

 but do not contain much car])onates. J. H. L. Vogt has shown that 

 along certain veins of Norway the granite and gneiss are altered to pro- 

 ducts resembling the "greisen " of the tin deposits. 



Origin of the Gold. 



Regarding the origin of the hot, auriferous solutions which have pro- 

 duced the goldnpiartz veins it is best, at this stage of our knowledge, to 

 speak with great reserve. P^ven the results of assays or analyses of 

 country-rock must be received with the greatest caution, to make sure 

 tliat tlie percentage discovered is i)rimary constituent and not later im- 

 pregnation. It is not to be denied that many reasons speak strongly 

 against a derivation from the surrounding rocks. Tlius, for instance, the 

 (horites of Nevada City and Grass Valley contain an appreciable amount 

 of l)arium, and still there is no trace of barite in the veins of those locali- 

 ties. In another instance the dialjasic rocks of the same region contain 

 copper, and yet the gold veins passing through these rocks are remark- 

 ably i)Oor in copper minerals. 



In discussing this difIicult<iuestion tiiereare several broad facts which 

 must be borne in mind : 



First, that the gold-<iuartz veins throughout the state of California are 

 closely connected in extent witli tlie.above-described metamorpliic series, 



♦ Monograph XIII, U. 8. Geol. Survey, p. 392. 



