R. Mallet — Reply to Mr. Poulett-Scrope. 131 



in this respect to Mr.. Scrope himself; over whom I have possessed this advantage, 

 whatever it may be worth, that my volcanic travels have been made not before, 

 but since, the state of thermotic science and of other branches of physics has 

 enabled measure and qiiantity to be applied to such phenomena. It is not 

 upon the vastness of field of observation, but by the combined "eye-sight 

 and insight" with which chosen portions of it maybe regarded that the interpretation 

 of natural phenomena depends. Von Buch and Humboldt afford us a remark- 

 able illustration of this. Both possessed almost unrivalled opportunities of volcanic 

 observation, yet to neither, so far as I am aware, do. we owe a single important 

 advance in volcanic theory. 



1 have nowhere denied, as Mr. Scrope- assumes, that the preponderant portion of 

 the ejecta constituting volcanic cones has been at so7ne time or other ina. siaXt of 

 fusion. What I have affirmed, and taken as part of the basis of my estimate, is, that 

 in any one eruptions, and upon the average of all known volcanoes, not more than 

 one-twentieth of the matter ejected is at that time reduced to the condition of 

 liquid fusion, the rest being merely more or less highly heated, but not to the fusing 

 point. I believe a majority of those who have best examined the subject will agree 

 with me in this. 



Mr. Scrope also objects- that I have not taken into account "the dust carried 

 away by the winds orj waves, and scattered over thousands of square miles of the 

 surrounding areas." But this again is not so. I have included all dust and 

 fragmentary matter in the nineteen-twentieths of ejecta heated to below fusing 

 point, and I believe most amply allowed for its mass. The mass of dust 

 carried to any considerable distance by the wind is relatively very small, 

 and of that carried away at present by sea-currents we know simply nothing. 

 Were I to admit Mr. Scrope's objections here as valid, and add as largely to 

 the heat expended in each eruption as he could show any reasonable ground 

 for, the iiicreased numerical result would not in the least degree invalidate the 

 argument of my paper, in which I have proved that the total amount of heat 

 annually carried off' from our globe by existing volcanic action cannot by any 

 possibility exceed the j^ part of the total heat annually dissipated from our globe. 

 I may extend this remark to nearly the whole of Mr. Scrope's objections in other 

 directions, which merely cavil with my numerical data, without supplying any 

 better or more exact ones, and which in any event do not affect my argument, or 

 the theory deduced from it. I must pass almost without notice Mr. Scrope's 

 energetic denials of my view, that on the whole the most ancient volcanic activity 

 observable on our globe's surface was hydrostatic, and not- explosive in its character, 

 as at present. Some of his objections rest on mere misconceptions of my views, 

 and all are asserted rather than proved ; and whatever view be taken as respects 

 this, does not affect the validity of my theory. The "unauthorized notion of the 

 existence of vast masses of ' dust ' beneath the earth's crust," which Mr. Scrope 

 attributes to me, and which, he says, "pervades much of my theoretical view of 

 the cause of ' hypogeal disturbances,' " has no existence but in his own imagina- 

 tion. I have nowhere even suggested the existence of any such masses save in 

 proximity to volcanic foci and vents, and from which, it is the province of the 

 volcano to dislodge them. 



The most sweeping- objections to my views urged by Mr. Scrope are to be 

 found at pp. 31, 32, commencing with the words "But the data for forming any 

 opinion," etc. The remarks here made by Mr. Scrope arise from a radical 

 misconception of the nature of my argument, the very basis of which he does 

 not seem to discern. Observations, long continued at Paris and Edinburgh, 

 prove that the annual loss of heat from our globe at present is equal to that 

 necessary to melt 777 cubic miles of ice at zero to water at the same temperature. 

 It is a result as well assured as most physical data dependent on continued obser- 

 vations, and sanctioned by the authority of such men as W. Thomson, the late 

 J. D. Forbes, Elie de Beaumont, etc. ; it is certainly not above the truth, but may 

 probably be considerably below it. Having ascertained experimentally the units of 

 heat evolved from the unit of volume of mean rock crushed, — that is, the mean of 

 the various rocks of the whole series of formations, — I am enabled to determine how 

 many cubic miles of such mean rock, if crushed, would evolve as much heat as that 

 annually lost by our globe. So far, the numerical data by which I have tested my 

 theory do not admit of dispute. The theory itself — viz. that volcanic heat and 



