84 BUXSEN ON THE FORMATION OF 



In consequence of these changes, the acid reaction of the water 

 with which the rocks are impregnated is converted into an alka- 

 line reaction, resulting from the formation of alkaline sulphurets 

 at the cost of the now alone-acting sulphuretted hydrogen. 

 Simultaneously with the disappearance of the acid reaction, 

 commences the action of the free carbonic acid upon the rocks, 

 and the alkaline bicarbonates resulting from this action provide 

 a solvent for the silica, by which means, and in accordance with 

 the most simple laws, which I have already described, those 

 wonderful geyser structures are formed, giving rise to the stu- 

 pendous phaenomena of the Icelandic eruptive springs. 



The springs of carbonic acid finally make their appearance as 

 the terminal stage of all this series of phaenomena. They gene- 

 rally survive the plutonic catastrophe the longest, and appear to 

 be exclusively limited to the Western Islands. 



The fumaroles on the craters of Hecla were, when I had an 

 opportunity of examining them more carefully shortly after the 

 eruption in 1845, in that state which I have described as being 

 the first stage of the secondary volcanic action. Not the slightest 

 trace of sulphuretted hydrogen could then be detected either by 

 the smell or by means of reagents, while, together with the 

 abundant sublimation of sulphur, the presence of sulphurous 

 acid could be recognized by its smell at a considerable distance 

 from the craters. On approaching a lighted cigar to the fuma- 

 roles, those thick clouds of smoke were indeed observed which 

 Piria has pointed out to be an indication of the presence of very 

 minute traces of sulphuretted hydrogen. However, as it is very 

 easy to ascertain that sulphur itself will cause the same phaeno- 

 menon when subliming with aqueous vapour, it is doubtful 

 whether even a trace of sulphuretted hydrogen accompanied the 

 exhalations from the crater at that time. In the year 1843, I 

 observed exactly the same phaenomena in the crater of Vesuvius, 

 when, after a long period of rest, it again began to show signs 

 of activity, and lava, solidifying to scoriae, commenced to pour 

 forth from the crater-cone with periodic explosions of vapour. 

 The last eruptions of the Krafla and Leiihnukr, during the last 

 century, likewise appear, according to the certainly very scanty 

 records preserved, to have been accompanied by such phae- 

 nomena. 



