Poulett- Scrope — On Mallet^ s Theory of Volcanic Energy. 29 



pose to call in question this theory, upon which I have in more 

 than one previous communication to this Magazine briefly ad- 

 verted.' 



The theory which is peculiarly Mr. Mallet's own, and is founded 

 upon the assumed truth of that just cited, finds the source of all 

 volcanic heat and consequent energy in the collision and crushing of 

 these subsiding rocks. Now the first remark to which this theory is 

 open is that it seeks and purports to find a second source of the heat 

 notoriously proceeding from the interior of the globe towards its 

 surface, and thence into outer space, where one already, ex hypothesi, 

 exists fully sufficient for the purpose. This outward escape of heat 

 from within the crust of the globe takes place in three modes, viz. : 

 1st, probably upon every point of its surface, by conduction through 

 the outer rocks, as proved by the nearly uniform downward increase 

 in temperature, perceptible in borings, wells, mines, and other ex- 

 perimental trials ; 2nd, by warm and often very hot springs that rise 

 to the surface ; and 3rd, by the escape of still more highly heated 

 water in the shape of steam, and also of fused rocks (lava) in 

 volcanic eruptions. Now Mr. Mallet admits that in the two former 

 of these modes, the escaping heat proceeds from tlie heated nucleus 

 below ; to which loss of heat, of course, he attributes the cooling 

 and consequent shrinking of that nucleus. But to the heat that 

 escapes in the third mode — that of volcanic eruptions — he ascribes a 

 totally distinct and independent origin, namely, the conversion into 

 heat of the crushing force to which the subsiding crust is, according to 

 him, subjected. The greater number of hot springs known to physical 

 geographers are, however, unquestionably found in close connexion 

 with volcanoes or volcanic rocks, some welling out from the craters 

 themselves ; and it is impossible to suppose that these owe their high 

 temperature to a source wholly different from that of the steam 

 which rises in and from the lava of the adjoining volcanic vent. 

 There is no difficulty in understanding how the great fissures in the 

 solid, crust of the globe, which are marked outwardly by trains of 

 active, or once active, volcanoes, may penetrate so far into the 

 interior of the heated nucleus as to give vent to an amount of heat 

 sufficient to fuse the rocks through which they pass, and to some of 

 the already fused or viscid underlying matter. 



I have more than once explained in the pages of this Magazine, 

 and elsewhere, in what mode it is probable that local inequalities of 

 temperature in the sub-cortical matter of the globe are produced 

 by heat, prevented from escaping through accruing thicknesses of 

 superficial stratified deposits of very low conductivity, accumulating 

 beneath them, and thence being drawn off laterally through the 

 more or less open rents which local variations of temperature, and 

 consequently of volume, must occasion in the matter underlying the 

 crust. '^ 



1 Geol. Mag. Vol. X. No. 7, July, 1873. 



2 See Geol. Mag. July, 1873, and March, 1870; also "Volcanoes," ed. 1862, 

 p. 308, and Mr. H. Woodward's Presidential Address to the Geological Association 

 for 1873. 



