Mallet's Theory of Vulcanicity. 45 



observed the gradual yielding of detrital rock-masses under pres- 

 sure, to understand why the cataclysmal yielding which mani- 

 fests itself in earthquakes should so frequently change its loeality 

 of occurrence — why for long periods a region may be completely 

 exempt from these movements, in consequence either of an un- 

 resisted and therefore gradual descent of the crust-fragments 

 underlying it, or of an arch-like arrangement, whose sudden 

 breaking down will result in a catastrophe, succeeded perhaps 

 by a long period of quiescence. 



Thus Mallet's theory accounts equally well for the sporadic 

 and apparently lawless occurrence of seismic phenomena, and 

 for the probable correlation between the frequency and violence 

 of earthquakes and tidal extremes. Unlike the theory of a thin 

 crust, which would lead us to expect almost diurnal earthquakes 

 corresponding to oceanic tides, according to Mallet's view there 

 should be a near coincidence in time and space of two indepen- 

 dent factors (viz. of a condition of very unstable equilibrium of 

 some crust-fragment, with a tidal extreme) in order to produce 

 a maximum of disturbance. It cannot be expected that such 

 coincidence should be of frequent occurrence, or that the casual 

 connexion should manifest itself in a greater predominance than 

 that claimed by Perrey for the times of spring and neap tides. 

 Mallet does not, however, allude to this point — whether from a 

 distrust of Perrey's data and method, or theoretical scruples on 

 the score of " rigidity." 



The objection, that according to Mallet's theory earthquakes 

 ought always to be followed by eruptions, could obviously 

 apply only during the period of fissure eruptions from the 

 liquid interior — it being conceded that the volcanic eruptions 

 of to-day are due to contact of water with the molten rock, and 

 that steam, not static pressure, is the vis a tergo. It is, of 

 course, very probable that the access of water to the volcanic 

 focus* is generally caused or facilitated by such crust-move- 

 ments as would at the same time result in the production of 

 more heat and perhaps of fused rock, such movements being 

 indicated by the (mostly slight) earthquakes that so frequently 

 precede a period of volcanic activity. Hutton's objection, that 

 according to Mallet's view each eruption ought to be preceded 

 by a sensible subsidence, is therefore groundless. 



One point, however, must strike every reader of the original 

 memoir, viz. the preeminence given by Mallet to the crushing of 

 solid rock as the means of producing heat and fusion. One 

 would naturally look to the results of his experiments on this 



* Hutton (loc. cit.) avers that "to cause a volcano the heat must go to 

 the water ; the water cannot go to the heat," but omits any explanation of 

 this singular axiom. 



