250 Br. T. Sterry Hunt — On Volcanic Action. 



that required for simple igneous fusion. The intervention of water 

 in giving liquidity to lavas, has, in fact, long been taught by Scrope, 

 and, notwithstanding the opposition of Plutonists like Durocher, 

 Fournet, and Eiviere, is now very generally admitted. In this con- 

 nection, the reader is referred to the Gteological Maga.zine for Feb- 

 ruary, 1868, page 57, where the history of this question is discussed. 



It may here be remarked that if we regard the liquefaction of 

 heated rocks under great pressure, and in presence of water, as a 

 process of solution rather than of fusion, it would follow that dimi- 

 nution of pressure, as supposed by Mr. Scrope, would cause not 

 liquefaction but the reverse. The mechanical pressure of great 

 accumulations of sediment is to be regarded as co-operating with 

 heat to augment the solvent action of the water, and as being thus 

 one of the efficient causes of the liquefaction of deeply buried sedi- 

 mentary rocks. 



That water intervenes not only in the phenomena of volcanic 

 eruptions, but in the crystallization of the minerals of eruptive rocks, 

 which have been formed at temperatures far below that of igneous 

 fusion, is a fact not easily reconciled with either the first or the 

 second hypothesis of volcanic action, but is in perfect accordance 

 with the one here maintained, which is also strongly supported by 

 the study of the chemical composition of igneous rocks. These are 

 generally referred to two great divisions, corresponding to what 

 have been designated the trachytic and pyroxenic types, and to 

 account for their origin, a separation of a liquid igneous mass beneath 

 the earth's crust into two layers of acid and basic silicates, was 

 imagined by Phillips, Durocher, and Bansen. The latter, as is well 

 known, has calculated the normal composition of these supposed 

 trachytic and pyroxenic magmas, and conceives that from them, 

 either separately, or by admixture, the various eruptive rocks are 

 derived ; so that the amounts of alumina, lime, magnesia, and 

 alkalies, sustain a constant relation to the silica in the rock. If, 

 however, we examine the analyses of the eruptive rocks of Hungary 

 and Armenia made by Streng, and put forward in support of this 

 view, there will be found such discrepancies between the actual and 

 the calculated results as to throw grave doubts on Bunsen's hypo- 

 thesis. 



Two things become apparent from a study of the chemical 

 nature of eruptive rocks ; first, that their composition presents such 

 variations as are irreconcilable with the simple origin generally 

 assigned to them, and second, that it is similar to that of sedimentary 

 rocks whose history and origin it is, in most cases, not difficult to 

 trace. I have elsewhere pointed out how the natural operation of 

 mechanical and chemical agencies tends to produce among sedi- 

 ments, a separation into two classes, corresponding to the two great 

 divisions above noticed. From the mode of their accumulation, 

 however, great variations must exist in the composition of the sedi- 

 ments corresponding to many of the varieties presented by eruptive 

 rocks. The careful study of stratified rocks of aqueous origin 

 discloses, in addition to these, the existence of deposits of basic 



