THE EXTRUSIVE PROCESSES. 619 



often given forth long after all signs of active vulcanism have disappeared. 

 Such gases have been attributed to the action of the lavas on buried 

 beds of hmestone or other carbonates, but in many cases the geology 

 of the region offers no special support to this hypothesis. It does not 

 seem inherently probable that the heat of the lava would be sufficient 

 to decompose limestone at a period very long after the active eruption. 

 An alternative suggestion is that the stronger volcanic acids mentioned 

 below are gradually conveyed into the adjacent rocks and there act 

 on limestones or on partially carbonated crystalline rocks, setting free 

 carbon dioxide. Whatever may be true with regard to secondary gases 

 of this kind, it is quite certain that the lavas themselves contain large 

 quantities of carbon dioxide, and also of carbon monoxide, doubtless 

 reduced from the dioxide. Sulphur gases are very common accom- 

 paniments of volcanic eruptions. They take the forms of sulphuretted 

 hydrogen and sulphurous acid and perhaps of sublimated sulphur, all 

 of which are liable to pass by oxidation and hydration into sulphuric 

 acid. Chlorine and hydrochloric gases are also common, particularly 

 at high temperatures. Fluorine and other gases are occasionally 

 present. Certain gases, such as hydrogen and chlorine, are especially 

 associated with high temperatures and energetic action, and are prob- 

 ably dependent on them. Hydrochloric acid and the sulphurous gases 

 are also mainly associated with high temperatures, while sulphuretted 

 hydrogen is commoner at lower temperatures. Oxygen, nitrogen, and 

 probably carbon dioxide or carbon monoxide are present throughout all 

 ranges of temperature. Nitrogen is a rather frequent but not very 

 abundant constituent of the volcanic gases. How far it results from 

 admixture of the atmosphere and how far it is original, is not determined. 

 It is, however, one of the gases found in volcanic rocks after they have 

 cooled, and is presumably original in part. A large series of secondary 

 vapors naturally arise from the volatilization of substances contained 

 in the lavas, such as the oxides, chlorides, and sulphides of the 

 metals, etc. 



Residual gases in volcanic rocks. — Some light upon the vital ques- 

 tion of the original, as distinguished from the secondary gases of lavas 

 may be found in the analyses of the gases that remain in the lavas after 

 they are solidified. When the lavas lodged underground without free 

 communication with the surface, there is reason to think that they 

 retained a larger percentage of their original gases in solidification than 



