Mallet's Theory of Vulcanicity. 4>7 



Taking for granted the correctness of Hirn's theorem, " that 

 the heat evolved in the crushing of rigid bodies is the equiva- 

 lent of the work performed/' Mallet's experiments on the con- 

 traction of fused rock in cooling, and his estimates of the amount 

 of volcanic energy manifested on the globe, coupled with that of 

 the earth's annual loss of heat, completed the proof of the quan- 

 titative adequacy of the cause invoked by him. And when it is 

 understood that the earth's present loss of heat during sixteen 

 and a half years is the mechanical equivalent of all the volcanic 

 work performed since the period of fissure eruptions, the burthen 

 of proof of the qualitative inefficiency of the several modes of 

 action that may come into play would seem to be effectually 

 thrown upon the opponents of the theory. 



Among these modes of action, the fusion of masses already 

 existing in a pasty, or generally more or less igneoplastic con- 

 dition, by squeezing or forcible displacement, seems to me to 

 deserve especial attention. At the depth at which volcanic 

 phenomena must be supposed to originate, this condition must 

 be closely approached, especially in the early times of the vol- 

 canic period — that of the " Maare " of the Eifel and other simi- 

 lar cases representing the transition phase between the regime 

 of fissure-eruptions and that of volcanoes proper. In this 

 period of a " greatly stiffened and thickened crust," even slight 

 flexures, whether synclinal or anticlinal, would occasion great 

 displacements and movements in the half-stiffened upper layers 

 of the " viscous couche /' and if these experienced local re- 

 fusion, the fused matter may well be presumed to have often 

 been disposed of by eruption through fissures or volcanic vents, 

 rather than by overcoming downward the inertia of the viscous 

 couches. This mode of action seems to me likely not only to 

 afford a more copious, but also a more constant or lasting source 

 of supply than the supposed crushing of solid rock, and appears 

 especially applicable to the case of large fissure-eruptions. 



Among the greatest services rendered by Mallet's (or, in this 

 connexion, Wurtz's) theory is the unstrained explanation of 

 many of the phenomena of metamorphism that were quite un- 

 intelligible so long as the heat required for the observed 

 changes was supposed to be derived from below, and perhaps 

 by transmission through strata which themselves had experi- 

 enced little or no change of condition. The principle that the 

 heat evolved in the flexure or forcible compression of strata is, 

 ceteris paribus, proportional to the resistance offered by them 

 to the external force, throws a flood of light upon numerous 

 apparently contradictory phenomena, which have long been 

 quoted as incompatible with the doctrine of metamorphism as 

 held in this country, and have stood in the way of its general 



