164 COSMOS. 



and the same internal force produces colossal craters of eleva- 

 tion, nay even the mighty, desolating, lava-pouring volcanoes 

 of Etna and the Peak of Teyde, and the cinder-erupting 

 Cotopaxi and Tunguragua. 



Amongst 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 tre- 

 mulous 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 production of any- 

 thing of a material nature. In the other phenomena 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 volcano 

 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 gene- 

 ralized, 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 vapour 

 to a solid condition, must present analogous phenomena. The 

 little that we know of the form of the moon's surface, ap- 

 pears to indicate this. 3 Upheaval and plastic activity in 

 the production of crystalline rock from a fused mass, are 

 conceivable even in a sphere which is regarded as destitute 

 of both air and water. 



The genetic connexion of the classes of volcanic pheno- 

 mena here referred to is indicated by the numerous traces 

 of the simultaneousness of the simpler and weaker with 

 stronger and more complex effects, and the accompanying 

 transitions of the one into the other. The arrangement of 

 the materials in the representation selected by me is justified 

 by such a consideration. The increased magnetic activity 

 of our planet, the seat of which, however, is not to be sought 

 3 Cosmos, vol. iii, p. 44; iv, pp. 426, 491, 495498. 



