B. Mallet — Reply to Mr. Poulett-Scrope. 127 



y. — In Eeplt to Mr. Scrope's Observations on Mr. Mallet's 

 Theory of Volcanic Energy. 



By Robert Mallet, C.E., F.R.S. 



OBSERVATIONS by Mr. Scrope on the subject to which he has so long 

 devoted his attention demand respectful attention, and that I should endeavour 

 to assign my reasons for not agreeing with the strictures which he has made upon 

 my paper, read to the Royal Society in June, 1872. 



I must disclaim the justice of Mr. Scrope's charge that I have thrown aside "super- 

 ciliously " the labours of all preceding geologists on the subject of Volcanic Energy. 

 I have never in my life consciously treated with other than respect the well- 

 directed labours, in whatever department, of any man of science. In the opening 

 sketch of my paper, which reviews past speculations as to the nature and origin 

 of volcanic heat, truth, above all things, demanded that I should point out the 

 baselessness of many of them. In doing this, I had the unpleasant task of point- 

 ing out that the speculations of geologists on this subject too often show an 

 ignorance or disregard of any sound appeal to the physical and mechanical 

 ■sciences while those of some mathematicians ; equally unfounded, because the 

 conditions actually existing in nature were ignored or set aside for others admitting 

 of more convenient treatment, were yet passed current amongst geologists, because 

 the latter either could not or would not decipher the symbols in which these un- 

 founded speculations are wrapt up. 



If I have appeared to underrate the views of geologists as to the nature and 

 origin of volcanic heat, it is because I believe them wholly untenable when tested 

 by the light of existing science. The older notion of the chemical origin of 

 volcanic heat and energy so long persevered in by Daubeny and others, in the 

 face of most obvious difficulties, at last died a natural death as a result of the 

 examinations to which volcanic ejecta have been submitted. That which succeeded 

 it — -the so-called mechanical theory, or that of a nucleus in liquid fusion beneath 

 an extremely thm solid crust — has already or is soon destined to give way under the 

 searching examination to which it can be now subjected by the present state of 

 thermotic and more especially of thermo-dynamic science. Mr. Scrope's own 

 notions, which involve that very thin crust and liquid nucleus, as most recently 

 formulated by him,* do not, I believe, materially differ from those formed and 

 enunciated by him some thirty years ago. At that time nobody, not even the late 

 Sir John Herschel, was enabled to test the validity of the notions current amongst 

 geologists as to the origin of volcanic heat. It is always an unpleasant shock to 

 admit the untenability of the views we have held and have even promulgated with 

 more or less authority for many years, yet the progress of science, so far as these 

 opinions may be unsound, compels us to do so, whether we will or no ; it has already 

 done so, I believe, as respects the notions still espoused by Mr. Scrope,* of an im- 

 mense liquid nucleus and excessively thin solid crust, as well as the notion of subter- 

 ranean fiery lakes, or a continuous liquid shell between the crust and nucleus. The 



[ * Mr. Mallet, in this and other passages, certainly entirely misapprehends Mr. 

 Scrope's views, since in several papers contributed by him within the last few years 

 to this Magazine he has expressly called in question the theory which Mr. Mallet 

 ascribes to him. For example, this is the chief purport of two papers having for 

 their title "On the Supposed Internal Fluidity of the Earth" [see Geol. Mag, 

 1868, Vol. V. p. 537, and 1869, Vol. VI. p. 145], and again in an article " On 

 the Cause of Volcanic Action" [see Geol. Mag. 1869, Vol. VI. p. 196] he con- 

 cludes his paper with these words : — " Since it has become the fashion of late 

 among the writers of popular geological treatises to assume as a matter of fact, 

 beyond dispute, that the substance of the globe, immediately beneath its thin 

 superficial crust (and probably to its centre), is in a state of fluid fusion, and that 

 the access of water from the sea above to this molten interior is the exciting cause 

 of earthquakes and volcanos, I have thought it well to express my reasons for 

 entertaining doubts, to say the least, as to the correctness of either hypothesis." 

 (p. 199.) Probably Mr. Mallet has never considered these papers. — Edit. Geol. 

 Mag.] 



