238 GEOLOGY OF THE COMSTOCK LODE. 



soifataric action. — The Only remaining supposition is that which connects 

 the heat of the Comstock with the chain of volcanic phenomena. What 

 is known as soifataric action is ill understood, and must remain so until 

 many of the mysteries of vulcanism have been made plain; but of certain 

 facts there is no doubt. In the neighborhood of active volcanoes, and often 

 also in regions where eruptions have ceased, gases and water charged with 

 more or less active reagents reach the surface through crevices. In its 

 earliest stages a soifataric spring frequently emits gas or water charged 

 with fluorine and chlorine compounds, which are replaced at a later stage 

 by hydrosulphuric and carbonic acids. The action of these reagents on the 

 rocks is manifold, but usually gives rise to characteristic appearances, such 

 as bleaching, accompanied by an extraction of a smaller or greater por- 

 tion of the bases. The appearances due to solfatarism are, of course, 

 accurately known, from immediate observation in the neighborhood of active 

 volcanoes. On the other hand, it is very seldom that effects likely to be 

 confounded with those of solfatarism are found at any great distance ft-om 

 localities marked by the occurrence, present or past, of volcanic eruptions. 

 No two phenomena in geology are more intimately connected than vol- 

 canoes and solfataras. The connection between ore deposits and eruptive 

 rocks is also in a large proportion of cases a very close one, and where 

 ore deposits and evidences of soifataric action are found together in a vol- 

 canic region, it is certainly natural to conclude that an abnormal temperature 

 of the rock and water is also due to vulcanism. The burden of proof 

 rests on him who offers any other explanation. 



Decomposed area at Washoe. — Extreme altcratiou is for the most part limited to 

 the area lying between the Comstock and the Occidental Lodes, though it 

 also extends up some of the ravines to the west of the great vein.^ Even 

 Avithin this area there are great variations in the degree of decomposition. 

 While a portion of the rock on the surface is tolerably well preserved, there 

 are belts nearly parallel to the Lode, in which it is so altered that it might 

 be mistaken for more or less discolored chalk. These belts can be followed 

 under ground, and retain in dip as in strike an approximate parallelism to 

 the vein. Towards the edges of the surface area it is common to find 

 nodules of rock in place which are fairly fresh at the center, but show pro- 



•See Fig. 1, page 73. 



