104 Q. Poulett Scrope — On Lavas. 



formed half a century back, and publislied almost as long; but 

 which, still ask for corroboration or disproof from more competent 

 obsei-vers. They have, I venture to think, an important bearing on 

 the history of the changes traceable on the surface of our planet, and 

 ought to be worked out, before any justifiable attempt can be made 

 to solve the problem of its sub-cortical character. 



P.S. — While the above was going through the press I have read 

 the paper on the Liquefaction of Eocks, by Dr. Sterry Hunt, in the 

 February number of this Magazine. My thanks are due to Dr. 

 Hunt for the generous manner in which on this, as on former 

 occasions, he has acknowledged, and even taken pains to vindicate 

 for me, such small merit as I may claim for the original advance- 

 ment of the suggestion — adopted by Dr. Hunt — as to the effect of 

 variations of temperature and pressure in promoting alternate lique- 

 faction and reconsolidation in a mass of mineral matter beneath the 

 crust of the earth. 



Dr. Hunt points my attention to a notice, by the Eev. 0. Fisher, 

 in the number of Scientific Opinion for October 27th last, which had 

 escaped my observation. In it Mr. F. expresses a doubt as to the 

 idea above refeiTcd to having been advanced in the first edition of 

 my work on Volcanos (1825), he having searched through that 

 volume without finding it. 1 beg, however, to refer him, or any one 

 whom the point may interest, to p. 26 of that volume, headed " Effect 

 of Increase of Temperature or Eeduction of Pressure." Mr. Fisher 

 will see, on reference to this and the following pages, that the 

 passage quoted by Dr. Hunt (p. 60 supra), from the edition of 1862, 

 only expresses, in more concise language, the idea here enunciated ; 

 which, indeed, forms the staple of my whole argument. 



Dr. Hunt, however, differs from me in one important respect, 

 belicAing that, under the supposed circumstances, the effect of 

 increased pressure would be exactly the reverse of that which I 

 suppose ; pressure, in his view, promoting, not preventing, lique- 

 faction, by " favouring the solution of the water-impregnated mass, 

 solution being, with few exceptions, a process of contraction." I do 

 not dispute the chemical theory on which this proposition is based, 

 and, indeed, the partial solution of some of the mineral constituents 

 of the subterranean matter (especially the silver) is involved in the 

 idea of its igneo-aqueous fusion. But it is not inconsistent with this 

 belief to suppose that the tendency to general liquefaction in the 

 mass, possibly occasioned by such partial solution under increased, 

 and the converse solidifying influence of diminished, pressure, may 

 be more than counteracted by the opposite tendency of such portions 

 of the super-heated interstitial water as exist at the time in the 

 state of vapour, to contract in volume in the first, and to expand in 

 the last case. It seems to me that all the phenomena of volcanic 

 action are best explained by this hypothesis, rather than by the 

 alternative which Dr. Hunt proposes. It is consistent with obser- 

 vation that as lava rises upwards in a volcanic vent, its efferves- 

 cence — that is, its tendency to liquefaction and elastic expansion — 

 increases with a diminution of pressure, even though so slight as 



