88 BUNSEN ON THE FORMATION OP 



mencement of this pneumatolytic process is not exclusively 

 limited to the more concentrated evolutions of vapour from the 

 solfataras ; on the contrary, it extends not unfrequently beyond 

 the widely-spread masses of trachytic rocks. It is more espe- 

 cially where these rocks penetrate the pyroxenic rocks, or are 

 penetrated by the latter, consequently in the nearest proximity 

 to the foci from whence, as I have shown above, the solfatara 

 gases originate, that all those characteristic indications of the 

 commencement of such fumarole actions make their appear- 

 ance. The yellowish or bluish gray colour of the trachyte is 

 replaced by a white ; the rock assumes a duller appearance ; 

 and even if the decomposition does not usually advance so far 

 that a perceptible loss of alkalies manifests itself, still a number 

 of small and chiefly microscopic crystals of iron pyrites and a 

 not inconsiderable per-centage of water may be detected in the 

 rock as characteristic indications of the already commenced ac- 

 tion of the solfatara gases. But these indications present them- 

 selves much more frequently, and in a more marked manner, in 

 the saalband of the trachytic dykes, where the metamorphoses 

 and abundant formation of iron pyrites in the adjoining rock 

 point out the course which was taken by these gases, whose 

 evolution was the secondary result of the great elevations of 

 trachyte. 



The basic palagonites and the pyroxenic rocks are still more 

 readily decomposed under the influence of the heated water and 

 the gases which it contains in solution than the acid trachytic 

 rocks are. The dark substance of the rock assumes in this case 

 also a lighter colour in the first instance, and breaks up into an 

 earthy mass, which gradually becomes richer in water and poorer 

 in alkaline bases and protoxide of iron, until finally it is entirely 

 converted into a white, bluish gray, yellow or red clay, filled 

 with small crystals of sulphur in beds, and not unfrequently con- 

 taining admixtures of gypsum. It sometimes happens that all 

 the stages of this metamorphism may be observed in one frag- 

 ment of pyroxenic rock taken from the spot where the solfatara 

 occurs. The frequently quite unaltered nucleus is found to pass 

 towards the exterior into a plastic argillaceous mass, which con- 

 sists of separate layers of white, gray, yellow, or brownish red 

 colour alternating with each other, sometimes containing iron 



