THfi VOLCANIC ROCKS OF ICELAND. 85 



In the extensive solfataras surrounding these latter two vol- 

 canoes, the second phase of the secondary volcanic action pre- 

 sents itself at the present time on the most stupendous scale, 

 while the volcanic activity is already in a state of considerable 

 decrease. As at Krisuvik, in the south-west of Iceland, the ex- 

 halations of sulphurous acid are very much inferior in magnitude 

 to the sulphuretted hydrogen, which here escapes from the 

 smoking clay soil and the boihng pools of mud in preponderating 

 quantity. 



With regard to the third phase of these operations manifesting 

 itself in the phaenomena of geysers, it can scarcely anywhere be 

 studied in its chronological relations more favourably than at 

 the famous springs of Haukadalr, which bear the name of the 

 Great Geysers. The crater of these springs, which has acquired 

 the material of its coating of sihceous tuff, like most of these 

 springs, from the readily decomposable palagonitic tuff, rests 

 upon a surface which is still in a state of fumarole action, situated 

 at the north-western edge of the spring cone, a section of which 

 is shown by a ravine. The streams of vapour, which there burst 

 through the fumarole clay, coincide most perfectly, in their ex- 

 ternal appearance and in their actions, with the springs which 

 are met with in the solfataras of Krisuvik and Reykjahlidh, with 

 the single exception that at the geyser there is no trace of sul- 

 phurous acid or any sensible deposition of sulphur. A glance 

 at the following composition of the gases, taken from this fuma- 

 role district, must, in fact, remove every doubt as to the identity 

 of the origin of all these phaenomena : — 



101. 



Nitrogen 84*11 



Carbonic acid 8*92 



Hydrogen . 6*59 



Sulphuretted hydrogen . . . 0'38 



Carbonic oxide 0*00 



Marsh gas 0*00 



Oxygen 0*00 



100-00 



The proportion of hydrogen to sulphuretted hydrogen and to 

 free carbonic acid, furnishes here also a standard for estimating 

 the consumption of the two latter gases, which, as may be seen. 



