ON THE CHEMISTRY OF METAMORPHIC ROCKS. 199 



at last little more than silica, alumina, and potash — the elements 

 of granite, trachyte, gneiss, and mica-schist. On the other hand 

 the finer marls and clays, resisting the penetration of water, will 

 retain all their soda, lime, magnesia, and oxide of iron ; and con- 

 taining an excess of alumina, with a small amount of silica, will 

 by their metamorphism, give rise to basic lime and soda-feldspars, 

 and to pyroxene and hornblende — the elements of diorites and 

 dolerites. In this way, the operation of the chemical and mechan- 

 ical causes which we have traced, naturally divides all the 

 crystalline silico-aluminous rocks of the earth's crust into two 

 types. These correspond to the two classes of igneous rocks, distin- 

 guished first by Professor Phillips, and subsequently by Durocher> 

 and by Bunsen, as derived from two distinct magmas ; which these 

 geologists imagine to exist beneath the solid crust, and which the 

 latter denominates the trachytic and pyroxenic types. I have 

 however elsewhere endeavoured to show that all intrusive or exotic 

 rocks are probably nothing more than altered and displaced 

 sediments, and have thus their source within the lower portions of 

 the stratified crust, and not beneath it. 



It may be well in this place to make a few observations on the 

 chemical conditions of mck-metamorphism. I accept in its 

 widest sense the view of Hutton and Boue, that all the crystalline 

 stratified rocks have been produced by the alteration of mechanical 

 and chemical sediments. The conversion of these into definite mineral 

 species has been effected in two ways : first by molecular changes ; 

 that isto say, by crystallization, and a re-arrangernent of particles ; 

 and, secondly by chemical reactions between the elements of the 

 sediments. Pseudomorphism, which is the change of one mineral 

 species into another, by the introduction, or the elimination of 

 some element or elements, presupposes metamorphism ; since only 

 definite mineral species can be the subjects of this process. To 

 confound metamorphism with pseudomorphism, as Bischoff, and 

 others after him, have done, is therefore an error. It may be 

 farther remarked, that although certain pseudomorphic changes 

 may take in some mineral species, in veins, and near to the surface, 

 the alteration of great masses of silicated rocks by such a process is 

 as yet an unproved hypothesis. 



The cases of local metamorphism in proximity to intrusive rocks 

 go far to show, in opposition to the views of certain geologists, 

 that heat has been one of the necessary conditions of the change. 

 The source of this has been generally supposed to be from below; 



