218 meunier: formation of magmatic gases 



respects, the laccolites, those products of aborted eruptions, 

 aborted because the siphon was not opened in time; that is to say, 

 no fissure was formed between the subterranean reservoir of 

 tensioned matter and the atmosphere, before sohdification. 



Thus, in order that a volcanic eruption prepared in the deeper 

 regions may reach the surface it is necessary that the lava capable 

 of gushing out shall find a passage from the subterranean labora- 

 tory to the surface. This passage "may be formed either by corro- 

 sion from below upward, or (more frequently) by the cracking of 

 the crust, for example, as the result of a seismic shock. 



It is hardly necessary to point out to what degree the preceding 

 observations explain the variants presented by the different volca- 

 noes as regards the composition of their magmatic gases. Some 

 rocks will furnish to the subterranean distillation nothing but 

 water, accompanied by a greater or less quantity of various second- 

 ary products; others, such as coal beds or carbonaceous schists, 

 will yield hydrocarbons or related compounds; others, containing 

 deposits of sea salt or various chlorides, will furnish hydrochloric 

 acid or its compounds; others, containing layers of gypsum or 

 similar deposits of sulphates and sulphides, will yield sulphuric 

 acid or sulphurous acid or sulphohydric acid. Elsewhere, 

 fossiliferous layers, filled, say, with animal remains, such as the 

 sapropelites of the oldest terranes, will yield ammonia and nitrog- 

 enous compounds. It is easy to imagine the infinity of cases 

 that may occur either separately or in association. 



In a word, the preceding theory seems adequate to explain the 

 different circumstances of the geographic distribution at each 

 epoch, the geologic position, and the chemical behavior of vol- 

 canoes. It recommends itself by its intimate connection with 

 all the other characteristics of planetary physiology, of which it 

 appears to be an inevitable consequence. 



