CONTACT METAMORPHISM. 491 



the so-called fumarolic and solfataric" actions may occur on an extensive 

 scale. In such regions fumarole is generally applied to the gases at the 

 higher temperatures, and solfatara to the gases at moderate temperatures, 

 although this distinction is not everywhere applied. Fumarolic and solfa- 

 taric action is therefore largely the work of gaseous solutions, although 

 liquid solutions are also at work to an important extent. The gases may 

 be widely dispersed through the porous rocks, but in many cases they 

 move chiefly along large definite channels, from which they spread for 

 various distances along smaller and less definite openings. 



Of the gases of fumaroles and solfataras, water vapor is dominant. In 

 many districts it is emitted in enormous quantities from the orifices. Prob- 

 ably the gas next in abundance to steam is sulphurous oxide (S0 2 ). Other 

 important gases are chlorine (Cl 2 ), hydrochloric acid (HC1), hydrofluoric 

 acid (HF1), hydrosulphuric acid (H 2 S), sulphuric acid (H 2 S0 4 ), carbon 

 dioxide (C0 2 ), oxygen (0 2 ), and hydrogen (H 2 ). Boric acid (H 3 B0 3 ), is 

 sometimes plentiful; nitrogen (N 2 ), is abundant; and hydrocarbons occur. 

 The proportions of these gases are very different in different localities. In 

 many cases the differences seem to be a function of the temperatures of the 

 fumaroles and solfataras. In the very hot fumaroles chlorine, hydrochloric 

 acid, and hydrofluoric acid are likely to be abundant. At lower temperatures 

 sulphurous oxide and hydrosulphuric acid are very common, and at still 

 lower temperatures oxygen, carbon dioxide, and nitrogen are abundant, In 

 the hot fumaroles, associated with the strong acids characteristic of them, are 

 also found many of the compounds characteristic of fumaroles and solfataras 

 of lower temperatures, although these may be overlooked. With all classes 

 of fumaroles and solfataras water vapor is dominant, but it is frequently 

 more markedly dominant at the lower than at the higher temperatures. 



The source of the gases of fumaroles and solfataras is a complex ques- 

 tion of which I shall not attempt to give an adequate discussion. It is to 

 be presumed, as pointed out on pages 931-937, that the ultimate source of 

 the various products formed in the belt of weathering is the material of 

 the original magmas. However, later it will be more fully explained that 

 several of the rarer elements of the igneous rocks are locally concentrated 

 in the belt of weathering by various processes. To what extent the active 

 gases of the volcanoes are derived from the original magmas and what 

 part from the later concentrations of these I shall not here consider. 



« Latin fumus, smoke, and Italian solfo, sulphur. 



