260 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



fissures in exactly or nearly the reverse quantitative proportion from that in which 

 they occur in silver veins. Iron and manganese are not only more abundant in 

 rocks, but also much more easily attacked and carried away by acids, than 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 copper ought to be more 

 subordinate, if their removal 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 silver 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." 



NATURE OF SOLFATARIC ACTION. 



Concerning the nature of solfataras, the following extracts are quoted from 

 Professor Bonney's Volcanoes (p. 52): 



"In the intervals between the paroxysmal phases most volcanoes emit simply 

 steam, and all in their decadence pass through a longer or shorter period when it 

 alone is ejected. This is often termed the solfatara stage, from the crater of that 

 name in the Phlegrsean Fields. Like most of those in this district, the cone is low 

 and the crater wide; the floor is a level, sometimes marshy, plain, surrounded by 

 steep walls of ashy materials, perhaps a hundred feet in height. The last eruption 

 was in 1189, when a stream of trachytic lava was discharged from the southern side 

 of the crater; but now the sole sign of activit}', except some boiling puddles in one 

 part of the floor, is to be found at the foot of the crag on the side. Here, from a 

 fissure in the inclosing wall, something like the adit of a mine, a column of steam is 

 ejected to a height of 6 or 7 yards. The steam commonly is more than the 

 vapor of water. Such acids as hydrochloric or sulphuric are often present;" that 

 of the solfatara, as we can see from the sulphur abundantly deposited round the 

 aperture and the rotten condition of the adjacent rocks, is no exception to the rule. 

 No doubt the materials in and about a vent must undergo considerable chemical 

 changes when the volcano is passing through this stage in its history." 



Professor Bonney finishes his summary of the description of volcanic eruptions 

 as follows (p. 62): 



"An eruption is generally ushered in by earthquake shocks, is always associated 

 with explosions, and is frequently concluded by the emission of a considerable mass 

 of lava. Great quantities of water are discharged in the form of steam, and the 

 phenomena of an eruption are closely imitated by geysers. Other vapors also are 

 discharged, and the solfatara stage of a dying volcano commonly ends with the 

 exhalation of carbonic acid or some such gas; perhaps the last stage of all may even 

 be a cold mineral spring." 



a The steam emitted from Vesuvius in January, 1876, was acid with these, particularly the former. Steel was rusted 

 and clothes were slightly altered in color in the course of an hour or two. 



