SALSES. 223 



If we now cast a last glance at the kind of volcanic 

 activity which manifests itself by the production of vapours 

 and gases, either with or without phenomena of combustion, 

 we find sometimes a great affinity, and sometimes a remark- 

 able difference in the matters escaping from fissures of the 

 earth, according as the high temperature of the interior, 

 modifying the action of the affinities, has acted upon homo- 

 geneous or very composite materials. The matters which 

 are driven to the surface by this low degree of volcanic 

 activity, are : aqueous vapour in great quantity, chloride of 

 sodium, sulphur, carburetted and sulphuretted hydrogen, 

 carbonic acid and nitrogen ; naphtha (colourless or yellowish, 

 or in the form of brown petroleum) ; boracic acid and alu- 

 mina from the mud volcanoes. The great diversity of these 

 matters, of which, however, some (common salt, sulphuretted 

 hydrogen gas, and petroleum), are almost always associated 

 together, shows the unsuitableness of the denomination 

 salses, which originated in Italy, where Spallanzani had the 

 great merit of having been the first to direct the attention 

 of geognosists to this phenomenon, which had been long 

 regarded as so unimportant, in the territory of Modena. The 

 name vapour and gas springs, is a better expression of the 

 general idea. If many of them, such as the Fumaroles, 

 undoubtedly stand in relation to extinct volcanoes, and are 

 even, as sources of carbonic acid, peculiarly characteristic of a 

 last stage of such volcanoes ; others, on the contrary, appear 

 to be quite independent of the true fiery mountains which 

 vomit forth fused earths. Then, as Abich has already shown 

 in the Caucasus, they follow definite directions in large tracts 

 of country, breaking out of fissures in rocks, both in the plains, 

 even in the deep basin of the Caspian Sea, and in moun- 

 tain elevations of nearly 8500 feet. Like the true volcanoes, 

 they sometimes suddenly augment their apparently dor- 

 mant activity by the eruption of columns of fire, which 

 spread terror all around. In both continents, in regions 

 widely separated, they exhibit the same conditions following 

 one upon the other ; but no observation has hitherto justified 

 us in supposing that they are the forerunners of the forma- 

 tion of true volcanoes vomiting lava and cinders. Their 

 activity is of another kind, perhaps originating at a smaller 

 depth, and caused by different chemical processes. 



