172 THE CANADIAN NATURALIST. [June 



ia fact, long been taught by Serope, and notwithstanding 

 the opposition of Plutouists, like Durocher, Fournet,and Riviere, 

 is now very generally admitted. In this connection, the reader is 

 referred to the Geological Magazine for February, 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 

 diminution of pressure, as supposed by Mr. Serope, 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 sedimentary 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 Buasen. 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 th^^ 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 calculated results as to throw grave doubts 

 on Bunsen's hypothesis. 



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 



