20 GEOLOGY OF THE COMSTOCK LODE. 



workiiifis will disclose such branches filled with vein matter, and probably 

 of ore-bearin<;f character, east of the main body of the vein. 



Alteration of minerals in situ. — A transforming actiou must necessarily take 

 place from the very commencement of the decomposition of matter in the 

 fissure. Sulphurous acid and sulphuretted hydrogen, Avhich were among the 

 escaping gases in the first period, together with combinations of fluorine 

 and chlorine, gradually became predominant, marking the second period of 

 solfataric action. But little more matter could be introduced into the fissure, 

 as the combinations of sulphur with mineral substances are not volatile. 

 Chemical transformation was now the principal action within the vein. Silica 

 is deposited from its combinations with fluorine and chlorine in a gelatinous 

 state, very difterent in its physical character from those of the crystalline 

 quartz which fills the vein. It must undergo a solution in water, with which, 

 in the form of steam, it was impregnated, in order to assume this character. 

 Metallic oxides and chlorides wove converted into sulphurets, and the pres- 

 ence of autimonv caused the formation of sul[)h-antimoniurets, the principal 

 one of which is stej)lianite. By such processes the entire vein matter was 

 o-radually converted from its former condition into that which it presents at 

 this day. 



Fluorine and chlorine. — It is a fact worthy of Hoticc that there is scarcely a 

 sino-le chemical agent, excepting fluorine and chlorine, which would not 

 carrv metallit' substances into lissures in exactl}- or nearly the reverse quan- 

 titati\e proportion from that in which they occur in silver veins. Iron and 

 mano-anese are not only more abundant in rocks, but also much more easily 

 attacked and carried away by acids tlian silver and gold. The proportion 

 of these to the former ought, therefore, to be still smaller in mineral veins 

 than it is in rocks, and lead and co[iper ought to be more subordinate, if 

 their renunal from their primitive place had been effected by other agents 

 than fluorine and chlorine. Only these two will first combine with those 

 metals which are most scarce in rocks, and relatively most abundant in sil- 

 \er veins; and they are probably the only elements which have originally 

 collected them together into larger deposits, though these may subsequently 

 have undergone considerable changes, and water may have played altogether 

 the most prominent part in bringing them into their present shape. 



