24 THE CHEMISTRY OF METAMORPHIC ROCKS. [III. 



way the operation of the chemical and mechanical 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 Duro- 

 cher 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 endeavored to show that all 

 intrusive or exotic rocks are probably nothing more than al- 

 tered and displaced sediments, and have thus their source with- 

 in the lower portions of the stratified crust, and not beneath it 

 (pages 4, 8 and 14). 



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

 the chemical conditions of rock-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 is to say, by crystalli- 

 zation, and a rearrangement 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 Bis- 

 chof, and others after him, have done, is therefore an error. 

 It may be further remarked, that, although certain pseudo- 

 morphic changes may take place 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 



* See further on this subject Essay XIII. and its appendix. 



