34 Notices of Memoirs — Mr. Prestwick — On 



and depths within the volcanic fissure ; ^ which seems to be much the 

 same thing, and is certainly no original idea. Again, Mr, Mallet 

 contumeliously rejects the common notion that volcanic eruptions 

 act as "safety-valves" to the force which would otherwise produce 

 destructive earthquakes.^ Yet he himself argues that his crushing 

 mechanism for producing the heat at intervals, which gives rise to 

 volcanic eruptions, obviates the occurrence of paroxysmal "Cata- 

 clysms " which would probably destroy all living things upon the 

 globe's surface. And what can be meant in this connexion by "Cata- 

 clysms" but earthquakes of tremendous violence? 



Mr. Mallet claims for his theory the special merit of explaining 

 the intermittent action of volcanoes, as well as the shifting of the 

 active vents from one point to another of the main volcanic bands 

 (§ 218). But these characteristics of volcanic action have been far 

 more reasonably accounted for by the fact that the violent discharge 

 of steam and lava during an eruption exhausts the energy of a 

 volcano for a time ; the lava sinking within the vent, and through 

 the outward loss of heat cooled down and caked over with a solid 

 crust, which for a time resists any further expansion by decrease of 

 heat from below — perhaps even seals up the vent so firmly that 

 the intumescent lava and vapoui find an easier issue in some other 

 more or less distant and weaker point of the main line of fracture 

 (See Yolcanoes, p. 41 et seq., 228, etc.) 



On the whole, while I admit the plausibility of Mr. Mallet's 

 suggestion that some local development of heat must attend the 

 crushing and squeezing of rocky matter during the internal move- 

 ments to which their fractures and contortions, as well as the slaty 

 cleavage of many, prove them to have been subjected, it appears to 

 me that the phenomena rather indicate as the true source of the 

 heat which has evidently occasioned the eruptions of both plutonic 

 crystalline rocks and of volcanic lavas, that intensely heated interior 

 (or nucleus), the existence of which is the first postulate of Mr. 

 Mallet's own theory. And to the lateral shifting of the wave of heat 

 outwardly transmitted from this source,- — shifting caused by the 

 varying impediments ofi"ered to the outward escape of this heat by 

 conduction through superficial deposits, — I prefer to ascribe the 

 internal movements that are observable alike in earthquakes and 

 volcanoes, in elevations and depressions of the crust, and extravasa- 

 tions of the internal heated matter. 



On the Geological Conditions affecting the Construction of a 

 Tunnel between England and ruANCE. By Joseph Prestwich, 

 E.E.S., F.G.S., Assoc. Inst. C.E. 



Being the substance of a paper read before the Institution of Civil Engineers on 

 Tuesday, December 9th, 1873. T. Hawksley, Esq., President, in the Chair. 



THE author, in this paper, reviewed the geological conditions of 

 all the strata between Harwich and Hastings on one side of the 

 Channel, and between Ostend and St. Yalery on the other side, with 

 1 § lU. * § 22i. 



