CHEMISTRY OF THE EARTH. 201 



ness, to give to a granitic mass a li(iui(lity partaking at once of tlie charac- 

 ter of an igneous aud an aqueous fusion. The studies by Mv. Sorby of 

 the cavities in crystals have led him to conclude that the constituents 

 of granitic and trachytic rocks have crystallized in the presence of liquid 

 water, under great pressure, at temi)eratures not above redness, and con- 

 sequently very far below 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 Iviviere, is now very geui'rnlly admitted. In 

 this connection, the reader is referred to the Geological Slagazine for Feb- 

 ruary, 1SG8, page 57, where the history of this question is discussed. 



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

 heated rocks under great pressure, and in presence of water, as a ])ro- 

 cess of solution rather than of fusion, it would follow that diminution 

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

 l)ut 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 sedimentary rocks. 



§ 42. 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 hypoth- 

 esis of volcanic action, but is in perfect accordance with the one here 

 maintained, which is also strongly supported by the study of the chem- 

 ical composition of igneous rocks. These are generally referred to two 

 great divisions, corresponding to what have been designated the trachy- 

 tic and pyroxenic types, (§ 27,) and to account for their origin, a separa- 

 tion of a liquid igneous mass beneath the earth's crust into two layers 

 of acid and basic silicates was imagined by Phillips, Durocher, and 

 Bunscn. The latter, as is well known, has calculated the normal com- 

 position of these supposed trachytic and pyroxenic magmas, and con- 

 ceives that from them, either separately or by admixture, the various 

 eruptive rocks are derived ; so that the amounts of alumina, lime, mag- 

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

 If, however, we examine the analyses of the eruptive rocks in 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 cal- 

 culated results as to throw grave doubts on Bunsen's hypothesis. 



§ 43. Two things become apparent from a study of the chemical na- 

 ture of erui^tive rocks : first, tliat their composition presents such varia- 

 tions 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. We have 

 already pointed out (§ 27) how the natural operation of mechanical and 

 chemical agencies tends to produce among sediments 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 sediments, 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 oi 

 deposits of basic silicates of peculiar types. Some of these are in great 

 l)art magnesian ; others consist of compounds like anorthite and labra- 

 dorite, highly aluminous basic silicates, into which lime and soda enter, 

 to the almost complete exclusion of magnesia and other bases ; while in 

 the masses of pinite or agalmatolite rock we have a similar aluminous 



