218 
ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
218. One cause for tlie extinction or long repose of volcanoes has been more or less 
perhaps recognized, they may get “ drowned out the activity is dependent upon a 
certain balance between the supply of heat and the supply of water ; if the latter become 
excessive, action dwindles to the solfatara or ceases altogether or for a time. But no 
prior theory assigns any cause for variability in the supply of heat : if that came from 
an immense liquid nucleus, it must be always the same and inexhaustible practically ; 
if from the imaginary lakes, it might he slowly exhausted, but could not rapidly decline 
nor suddenly vanish, unless by the draining dry of its contents or of the heat of the 
whole lake. 
On our hypothesis, however, avc find an adequate explanation for the sudden produc- 
tion at a given spot and for the rapid exhaustion of the source of heat, and for its pos- 
sible non-production again at the same spot or its production at another, adjacent or 
remote ; in other words, for the long observed shifting of the position of volcanic vents 
in the course of time, as well as for the secular enlargement of the superficial areas 
within which their action occurs, as remarked by Humboldt (Cosmos). 
219. Lastly, it presents a complete solution to the question, "Why should volcanoes 
present the linear arrangement they do on our globe, and why should they on the whole 
follow the lines of great mountain-chains 1 As manifestations of a common cause, con- 
traction by secular cooling, but different in degree, the mountain-ranges heaped up by 
tangential pressure (as has been already stated) have been formed along lines of con- 
trary flexure and of great Assuring and weakness in the earth’s crust. But it is along 
such lines of weakness that the crushing by tangential pressure of the cooler and more 
rigid crust must principally occur ; so that we may admit that at present the entire, or 
nearly so, of the tangential pressures produced by secular cooling are balanced by 
crushing limited to those great lines. 
Hence the line of volcanic vents follows the mountain-chain : — 1st, because there the 
fissures and vents of a shattered crust are chiefly found ; 2nd, because it is in such lines 
that the chief crushing goes on as being the weakest places, so that there the heat for 
the volcano is provided. Here the “ lake theory ” signally fails. On what conceivable 
grounds shall we imagine these imaginary lakes arranged beneath great curved lines on 
the surface, as beneath the immense line of volcanoes that girdles the Pacific 1 
Why, if produced as residues of a frozen liquid nucleus, should they not rather be 
scattered pretty evenly in a couche beneath the whole surface of the earth \ But if they 
be so, then the greater part of them must be hermetically sealed up from water, or, by 
hypothesis, we should have volcanoes dotted all over the earth ; and this last is equally 
true as applicable to Sciialee’s liquid zone or a universal liquid nucleus. 
220. Along such lines of volcanoes one vent or another may start into activity, 
according as the crushing energy beneath supplies more heat and pulverized material 
than another ; and we find an explanation for the observed (or supposed to be observed) 
fact without calling in the very crude notion that one volcanic vent relieves another, all 
drawing from a common fiery ocean or from lakes beneath. 
