On the Agency of Water in Volcanic Eruptions. 119 



extremely minute pores, or was condensed by pressure and refrigera- 

 tion previously to the solidification of the matter."* 



It must also be borne in mind that for this hypothesis to have any 

 value, the explosive material must extend throughout the mass of lava 

 and act from its base upwards, just as much as it is necessary that the 

 powder in the breech of the gun should be at the back of the shot. 

 It should therefore extend to the volcanic foci, at whatever depth 

 that might be, and be there occluded in the lava. 



Professor Judd, in his excellent summary of Mr. Scrope's views, 

 remarks that on this hypothesis volcanic outbursts are considered to 

 be ; ' due to the accumulation of steam at volcanic centres, and that 

 the tension of this imprisoned gas eventually overcomes the repressing 

 forces which tend to its manifestation," and that *' in the expansive 

 force of great masses of imprisoned vapour, we have a competent 

 cause for the production of fissures through which volcanic outbursts 

 take place. "f 



Mr. Scrope does not enter upon the question of the mode in which 

 the water has become occluded in the fluid magma. Sir Charles Lyell, 

 however, in supporting the views of Mr. Scrope, says, " We may 

 suppose that large subterranean cavities exist at the depth of some 

 miles below the surface of the earth in which melted lava accumu- 

 lates, and when water containing the usual mixture of air penetrates 

 into these, the steam thus generated may press upon the lava and 

 force it up the duct of a volcano, in the same manner as a column of 

 water is driven up the pipe of a geyser. "J 



Briefly the opinion of Mr. Scrope upon the cause of volcanic action 

 is that it is to be attributed to the escape of high pressure steam 

 generated in the interior of the earth. Before proceeding to discuss 

 this hypothesis more fully, and to state my objections to the views of 

 this distinguished volcanologist, I must refer to the remarkable paper 

 of the late Mr. R. Mallet, in which the same subject is treated from 

 an entirely different point of view. 



According to Mr. Mallet, volcanic energy, as we see it on the globe, 

 is not the direct product of primordial heat of fusion, although it is 

 evidently due to the loss of that heat, and is the result of the cooling 

 of our globe. He defines it thus : " The heat from which terrestrial 

 volcanic energy is at present derived is produced locally within the 

 solid shell of our globe by transformation of the mechanical work of 

 compression or of crushing of portions of that shell, which com- 

 pressions and crushings are themselves produced by the more rapid 

 contraction, by cooling, of the hotter material of the nucleus beneath 

 that shell, and the consequent more or less free descent of the shell by 



* Op. cit., pp. 36-7. 



f Judd's " Volcanoes," pp. 33 and 189. 



X " Principles of Geology," 10th Edit., vol. ii, p. 221. 



