158 COSMOS. 



its surface* To this entirety I give the general name of 

 Vulcanism or Vulcanicity ; and I regard it as advantageous 

 to avoid the separation of that which is causally connected, 

 and differs only in the strength of the manifestation of force 

 and the complication of physical processes. By taking this 

 general view, small and apparently unimportant phenomena 

 acquire a greater significance. The unscientific observer 

 who comes for the first time upon the basin of a thermal 

 spring and sees gases capable of extinguishing light rising 

 in it, or who wanders amo*ng rows of changeable cones of 

 mud volcanoes scarcely exceeding himself in height, never 

 dreams that in the calm space occupied by the latter erup- 

 tions of fire to the height of many thousand feet have often 

 taken place ; and that one and the same internal force pro- 

 duces colossal craters of elevation nay, even the mighty, 

 desolating, lava-pouring volcanoes of ^Etna and the Peak of 

 Teyde, and the cinder-erupting Cotopaxi and Tunguragua. 



Among the multifarious, mutually intensifying phenomena 

 of the reaction of the interior of the earth upon its external 

 crust, I first of all separate those the essential character of 

 which is purely dynamical, namely, that of movement or 

 tremulous undulations in the solid strata of the earth ; a 

 volcanic activity which is not necessarily accompanied by any 

 chemical changes of matter, or by the expulsion or produc- 

 tion of any thing of a material nature. In the other phe- 

 nomena of the reaction of the interior upon the exterior of 

 the earth in gas and mud volcanoes, burning springs and salses, 

 and in the large burning mountains to which the name of vol- 

 cano was first, and for a long time exclusively applied, the 

 production of something of a material nature (gaseous or 

 solid), and processes of decomposition and gas evolution, 

 such as the formation of rocks from particles arranged in a 

 crystalline form, are never wanting. When most fully gen- 

 eralized, these are the distinctive characters of the volcanic 

 vital activity of our planet. In so far as this activity is to be 

 ascribed, in great measure, to the high temperature of the 

 innermost strata of the earth, it becomes probable that all 

 cosmical bodies which have become conglomerated with an 

 enormous evolution of heat, and passed from a state of vapor 

 to a solid condition, must present analogous phenomena. 

 The little that we know of the form of the moon's surface 

 appears to indicate this.f Upheaval and plastic activity in 



* Cosmos, vol. i., p. 202-204. 



t Cosmos, vol. Hi., p. 44; iv., p. 104, 151, 154-156. 



