XIII] ORIGIN OF CRYSTALLINE ROCKS. 315 



crystalline forms which, though totally different in their origin from 

 the products of chemical alteration or substitution, are emphatically 

 pseudomorphs. 



From this process of mechanical and more or less heterogeneous 

 envelopment, Delesse next proceeds to consider the crystallizing 

 together of isomorphous or homceomorphous species, in relation to 

 the generally received notion of epigenic pseudomorphism. He 

 declares that " isomorphism explains very well facts which are 

 often attributed to pseudomorphism," and that many "minerals 

 which are still considered pseudomorphs are in reality examples of 

 isomorphism" (pages 364, 365). Referring to the well-known in- 

 vestigations of Mitscherlich upon the crystallizing together, in all 

 proportions, of isomorphous species, and of the symmetrical crys- 

 tallization of one salt around a nucleus of another isomorphous 

 with it, Delesse suggests that the different forms and varieties of 

 hornblendic and pyroxenic minerals afford many examples of the 

 kind. He then adds, " If, as Scheerer has remarked, water plays in 

 silicates the part of a base, anhydrous silicates may crystallize at 

 the same time with hydrated silicates, and, moreover, be isomor- 

 phous with them." In this way, he suggests, we may explain by 

 isomorphism or homo3omorphism, the association with pyroxene 

 of the hydrous species, schiller-spar, as well as that " of various 

 anhydrous and hydrated minerals" (pages 357, 358). 



In further illustration of the words just quoted from Delesse, we 

 may cite from Scheerer, as examples of what he called polymeric 

 isomorphism, the association (in the same crystals) of iolite and 

 aspasiolite, and of chrysolite and serpentine. If these and similar 

 species crystallize together because they are isomorphous, it is 

 evident that they may each crystallize separately ; and thus the 

 crystals of serpentine with the form of chrysolite, and those of 

 aspasiolite and other so-called hydrous iolites, may be regarded as 

 examples, not of epigenesis, but of isomorphism. 



We have thus endeavored to set forth, chiefly in his own words, 

 the views enunciated in 1859 by Delesse, according to whom the 

 phenomena of so-called pseudomorphism among mineral silicates 

 are to be explained, for the most part, not by chemical alterations 

 of pre-existing species, but by envelopment and by isomorphism. 

 That the above are really his views, and are, moreover, regarded by 

 himself as contrary to those of the school which I oppose, Delesse 

 does not permit us to doubt ; for, after having set them forth as his 

 own (aprls avoir expose notre manure d& voir), he says, " We hasten 



