90 BUNSEN ON THE FORMATION OF 



a number of large boiling pools of mud, which throw up their 

 blackish gray argillaceous contents to a height of 10 or 15 feet, 

 and heap it up in circular crater-like enclosures. One of these 

 pools still shows at some depth traces of the adjacent pyroxenic 

 lava stream, which bounds the solfataras on the north-east, and 

 whose decomposed substance forms the argillaceous mud which 

 fills the pool. The following analysis of this mud shows that 

 lava rocks suffer the same decomposition as the palagonite when 

 exposed to the action of the solfatara gases : — 



106. 



Silica 55-62 



Alumina 12*77 



Peroxide of iron . . 1*91 



Lime 1*56 



Magnesia .... 036 



Potash 0-43 



Soda 1-18 



Water 5*53 



Sulphur 0-92 



Sulphate of lime . . 3*45 



Iron pyrites . . . 16*27 



100-00 



When we find the solfatara action lasting for centuries afler 

 the comparatively insignificant eruptions of modern volcanoes, 

 we may certainly expect to meet with traces of similar processes 

 resulting from the far more stupendous catastrophes which gave 

 rise to the formation of the older trachytic and pyroxenic rocks. 

 And in reality these may be observed, with all their characteristic 

 peculiarities, in the plutonic dykes or injected masses occurring 

 in Iceland, and traced from thence under circumstances which 

 do not leave the least possibility of entertaining the remotest 

 idea of their being the results of changes which rocks suffer 

 under the influence of a simple process of extraction by surface 

 water. The pneumatolytic changes, indeed, frequently take place 

 without any removal of constituents from the altered rocks. 

 The pyroxenic dyke masses, passing in their saalbands into a 

 clay mixed with pyrites, carbonate of lime and gypsum, present 

 themselves in a state which does not admit of their being distin- 



