86 BUNSEN ON THE FORMATION OF 



is far greater in this instance than in the solfataras. These few 

 simple and easily-intelHgible processes of volcanic gas formation 

 contain the key to a whole series of metamorphic changes, which 

 may be comprised under the name of the pneumatolytic, as a uni- 

 versal and frequently-occurring class of phaenomena. The com- 

 position and mode of formation of the acid and alkaline thermal 

 waters of Iceland, which may easily be ascertained even by direct 

 experiment, follow as simple consequences from these processes ; 

 and the argillaceous formations, which are found on the saal- 

 bands of the trachytic and pyroxenic dykes, but on the most 

 extended scale in the beds of conglomeritic amygdaloids and 

 tuffs, penetrated by plutonic rocks, have in part resulted only 

 from a stupendous repetition of those very same processes of 

 decomposition, which we may daily observe going on at the 

 surface of the Icelandic solfataras. Without here referring at 

 length to the less interesting modes of rock formation, which are 

 determined by the separate action of aqueous vapour and hydro- 

 chloric acid, I will restrict myself, in order not to exceed the 

 limits of this paper, to a brief statement of some few alterations 

 of rocks which take place under the influence of the solfatara 

 gases, and with which the long series of rock metamorphisms 

 concludes. While the palagonitic metamorphosis gives rise to 

 the formation of hydrated products, in w^hich the relative pro- 

 portion of the normal pyroxenic mass appears scarcely altered, 

 the pneumatolytic metamorphism is accompanied by a loss of 

 substance, by the decomposed rocks extending in the first in- 

 stance to the alkalies and alkaline earths, and then to the oxides 

 of iron and the silica. The influence of the fumarole gases, 

 which no volcanic rock is able to withstand, not even the most 

 acid trachyte, may be traced through all the phases of a pro- 

 gressive decomposition in the rocks of the solfatara districts. 

 The first commencement of this action manifests itself as a 

 bleaching of the rock ; the dull appearance thus acquired is then 

 followed by a disintegration of the mass, which advances until 

 the rock becomes easily pulverizable ; and finally, when the 

 action is completed, there remains a plastic argillaceous mass 

 almost entirely free from iron, which perfectly resists any further 

 change, and after drying presents a very friable character, and 

 gives a bright streak. The characteristic collateral products of 



