R. Mallet— Reply to Mr. Poulett-Scrope. 133 



except the last, of little certainty. The limits of difference have never been 

 even analogically assigned by any physicist, and they are certainly smaller as the 

 rigidity is greater and the fusing temperature of the body is higher. The 

 notion -was seized upon by Hopkins, with but little examination, as offering some 

 feeble support to his wild hypothesis of subterranean lava lakes. 



Mr. Scrope thinks that my theory "fails to account for the fact that volcanic 

 eruptions are almost wholly confined to certain lines or bands traversing the earth's 

 surface," which, he goes on to say, "^indicate the existence through long geologic 

 ages of great rents in the solid crust, the direction of which is generally parallel 

 to the coast outlines of the continents or the axes of their mountain ranges. " (p. 32. ) 

 My theory, as pointed out in my paper, does adequately account for the arrange- 

 ment of volcanoes such as we find them, because it is along these bands that we 

 can see must have existed the lines of least resistance to crushing after the moun- 

 tain elevatory work had been done. Let me ask, on the other hand, what rational 

 solution of the observed arrangement of the volcanic and accompanying 

 seismic bands on our globe, which are not always coast-lines, but often cross the 

 seas and oceans^ is afforded by either the old, and as I regard it exploded, notion 

 of a universal ocean of molten lava beneath an excessively thin crust, or by 

 Hopkins's hypothesis of fiery lakes scattered within a solid and very thick crust ? 

 If we had the universal ocean of molten lava within thirty or sixty miles of the 

 surface, why should it confine its visits to the latter to any linear arrangement ? 

 Why should not the greater number of volcanoes be found about the tropics, where , 

 upon any theory of cooling of our globe, the crust, no matter how thick, must be 

 the thinnest ?' And why should we have any volcanoes at all about the neighbour- 

 hood of the Poles ? Or, again, why should Mr. Hopkins's fiery lakes be arranged in 

 lines, or in any other way, unless scaXter&d par sejiie awer land and sea bottom? It is 

 absurd to discuss the possibility of existence of the deep open fissiires imagined by Mr. 

 Scrope along coast-lines or anywhere else in our globe, after what the Rev. O. 

 Fisher and I have proved as to the enormous tangential pressures existing in the 

 earth's crust, which must crush into contact the walls of all such fissures with a 

 force nearly 500 times greater than the resistance of solid granite or porphyry. 



Mr. Scrope remarks that I " follow those geologists, Lyell, etc., who consider 

 eruptions to be occasioned by the influx of water from seas or lakes above through 

 fissures into foci of heated lava below. And that I reject as wholly untenable 

 the notion that water could have originally existed in molecular combination with 

 the crystalline matter of the rocks before they were melted into lava." Surface 

 water must in some way reach volcanic foci to account for the phenomena observed 

 at volcanic vents, and the theory held by Mr. Scrope, whether originating witli 

 him or not, and by a few other geologists, that the water from which the steam 

 issuing from volcanic vents is formed has been derived from water either chemically 

 combined or vesicularly contained in the rocks from which the lava has been 

 formed, is wholly untenable. It follows, therefore, that Mr. Scrope's favourite 

 notion that the expulsion of volcanic ejecta is due to the expansion by heat of such 

 combined or vesicular water, and that the lava rises in and is expelled from the 

 vent by what he calls its '' intumescence," by a process which he has himself 

 likened to the frothing forth of a bottle of champagne, is utterly untenable, 

 being inconsistent alike with the phenomena and with the physical laws upon 

 which it is supposed to be founded. If Mr. Scrope will recux to paragraphs 210 

 to 218 inclusive of my paper, I think he will see that he has overlooked much 

 that I have there said, and in part (no doubt unintentionally) represented the very 

 opposite of my meaning. So far from admitting that there is no limitation to the 

 depth to which water may percolate, I distinctly state that "it is only to such 

 depth as water can percolate or infiltrate by capillarity that the deepest focus 

 of volcanic activity can be found." (par. 211.) Nor, of course, do I deny that 

 rocky masses may be fused in contact with water at a sufficient temperature 

 and pressure. But I do deny that such water can be the source from which the 

 steam of volcanic eruptions is derived. 



I can but glance at the objections to Mr. Scrope's view. To treat it thoroughly 

 would require a whole part of this Journal or more. What does any geologist 

 kitow of the rock thirty or sixty miles deep ? That it is hydrated or hydro- 

 ferrous at these or muchi greater depths, not to say that the still deeper molten 

 rock of the nucleus is so, is a mere assumption. But let us admit that it all, 



