74 BUNSEN ON THE FORMATION OF 



It is very easy to ascertain that the rock formed from the lava 

 which flowed in 1845 from Hecla^ and which yielded the subli- 

 mates frequently impregnated with traces of free hydrochloric 

 acid, itself contains a sensible quantity of basic chlorine com- 

 pounds in its mass. 100 parts of lava from the eruption crater 

 contained in fact 0*246 chlorine, and the same quantity taken 

 from the end of the stream 0*447. 



From the nature of the origin of hydrochloric acid fumaroles, 

 it necessarily follows that they present the character of perma- 

 nent phaenomena only when the direct volcanic activity, together 

 with the degree of temperature indispensably necessary for this 

 processf*has not receded to any considerable depth below the 

 surface ; for otherwise the hydrochloric acid acting so powerfully 

 upon the substance of the rocks, would very soon give rise to the 

 formation of chlorides which are destitute of that degree of vola- 

 tility which would enable them to reach the surface under the 

 influence of a moderate temperature. It is for this reason that 

 fumaroles are observed as the direct results of great volcanic 

 eruptions soon after they cease, and continuing in a permanent 

 state of activity only where lava has reached the surface in the 

 form of scoriaceous ejections, which have lasted for a consider- 

 able time. 



If these conditions are wanting, the seat of the change recedes 

 further below the surface, where it may long continue in a state 

 of activity in foci of mineral waters, as may even be inferred 

 from the composition of thermal spring waters of Iceland, with 

 regard to whose formation there cannot for one moment be a 

 doubt, as they may easily be made artificially by the action of 

 the volcanic gases upon the Icelandic rocks. 



The sulphuretted fumaroles, on the contrary, have an entirely 

 different origin, and are far less ephemeral, lasting sometimes for 

 centuries afler the great volcanic eruptions. Iceland presents, 

 in its stupendous solfataras and geysers, which are the two prin- 

 cipal stages of this fumarole process, the most abundant material 

 for its thorough investigation. Nevertheless, I must in this 

 place restrict myself to the statement of those results of my in- 

 vestigations upon this subject which relate to the general con- 

 nexion of these phaenomena with the original processes of vol- 

 canic activity. 



