at Sulphur Bank, California. 33 



coming subterranean waters. We now come to speak of the 

 action of the down-going surface waters. The deposit of sul- 

 phur last spoken of forms the transition between the two, since 

 it requires the presence of air. 



Action of descending surface tout- ,•■-: i a i Tin' sulphuric acid 

 is, of course, formed by the more complete oxidation of the 

 siilphydric acid gas, the result of more complete contact with 

 air. In fact this takes place mainly in the air, and the effect 

 of prevailing dry winds in carrying it in particular directions is 

 plainly seen. Sulphuric acid is however also formed as a 

 secondary product in the oxidation of pyrites, (b) Iron Sul- 

 phate is first formed by oxidation of pyrites and then by further 

 oxidation runs down to (c) ferric end nt<<.jn>ljc <,, ide, giving up 

 its sulphuric acid. Tl either hy 



direct oxidation of H 2 S, or set free from iron sulphate by oxi- 

 dation of the latter, attacks the iron and alumina, in fact all the 

 bases of the volcanic rock forming iron sulphate and (d) 

 alumina sulpha!*: and leaves the rock as (e) a snow-white pow- 

 dery mass of pure silica. 



These surface effects of solfataric waters are seen in many 

 places in the Clear Lake region as, for example, at Chalk Moun- 

 tain, several miles north of Sulphur Bank, and elsewhere. 

 Somewhat similar effects have been observed also in other parts 

 of the world where solfataric springs occur. But the deeper 

 effects have been less observed and are therefore of the 



We have attributed the deposit of metallic sulphides and of 

 silica, to cooling and relief of pressure and, possibly, escape of 

 H 2 S. Doubtless these are the main causes of decrease of 

 solvent power and therefore of deposit. But it is probable also 

 that at a certain line where the up-coming alkaline meet the 

 down-going acid waters, the deposit is completed by neutraliza- 

 tion. Thus the line of demarkation between the two kinds of 

 reaction is sharper than it would otherwise be. 



It will be observed that in the present article we have con- 

 fined ourselves wholly to the description of phenomena and to 

 such immediate explanation as is forced on the observer. We 

 have not attempted any discussion of the bearing of these facts 

 upon the general theory of metalliferous vein formation. This 

 is reserved for a possible future paper in case our investigations 



Am. Jour. Sci.— Tihbd Series, Vol. XXIV, No. 130.— Jclt, 1882. 



