THE VOLCANIC ROCKS OF ICELAND. 83 



Sulphuretted hydrogen . . . 93*99 

 Hydrogen 6*01 



100-00 



The phaenomena upon which depends the activity of solfataras 

 become, after these experiments, very easily intelligible. It is 

 well known that almost all volcanic eruptions are accompanied 

 by sublimation of sulphur. Consequently the zone from whence 

 the sulphurous acid originates is situated where such masses of 

 sulphur, whose occurrence may be easily accounted for by the 

 action of volcanic heat upon decomposable sulphur compounds, 

 come into contact in the state of vapour with red-hot pyroxenic 

 rocks. When, at a subsequent period, the temperature commu- 

 nicated to this zone by the volcanic action sinks, a new phase 

 of chemical action commences. The sulphur compounds of iron, 

 and perhaps likewise of alkaline and earthy metals, which have 

 been formed there, commence their action upon the aqueous 

 vapour, and from this mutual action result sulphuretted hydro- 

 gen and the products of its decomposition — free hydrogen and 

 sulphur vapour. It is therefore evident that these two processes 

 are blended in each other, and meet in such a manner as neces- 

 sarily to determine the irregular simultaneous exhalation of those 

 gases at spots but little distant from each other in the same 

 fumarole district. These processes likewise afford an explana* 

 tion of the chronological course of the fumarole action. The sul- 

 phurous acid, whose appearance alone characterizes the initial 

 stage of all these phaenomena, is accompanied after a while by 

 sulphuretted hydrogen, which by its reaction with the former 

 gas gives rise to that succession of decompositions which cha- 

 racterize the true solfataras. Acid liquids saturate the rocks 

 traversed by separated sulphur and torn up by aqueous vapour, 

 converting these rocks, as I have already shown, whether they 

 belong to the pyroxenic or trachytic group, into clay, by ex- 

 tracting from the silicates potash, soda, magnesia, lime, protoxide 

 of iron, and frequently a part of the alumina, as sulphates. 



This phase of destructive action is followed, in course of time, 

 by a productive one, which increases in proportion as the source 

 of sulphurous acid becomes extinct, and the gradually decreasing 

 evolution of sulphuretted hydrogen recedes to greater depths. 



G2 



