278 EXPERIMENTS ON METAMOEPHISM AND 



crystallized silicates of alumina, as chiastolite and staurotide, in the 

 midst of pliyllades containing fossils, as well as that of garnet, 

 pyroxene, or feldspar in limestone, also of sedimentary origin, which 

 very often are not sensibly modified. Heat, and the crystallization 

 which is a consequence of cooling, might, it is true, cause a separa- 

 tion or liquation between substances which were j^rimitively dis- 

 solved, the one in the other. It is thus that carbon is separated in 

 crystals from cast iron in the state of graphite. But direct experi- 

 ment shows us nothing analogous to the development of isolated 

 crystals of garnet, pyroxene, feldspar, and disthene, by the agencies 

 of heat, in a calcareous gangue, which has not even been softened, 

 and which, according to all appearances, has been very feebly heated.* 

 We can conceive that the slow agencies which nature often employs 

 to form mineral products are perfectly capable of producing results 

 which man is entirely unable to imitate; but have we the right to 

 seek for explanations which there is nothing else to justify, exclu- 

 sively, in the duration of time and in vague causes which are, so to 

 speak, occult? 



The same mineral is often found entirely isolated and crystallized 

 in very different matrices; tourmaline, mica, feldspar, garnet, and 

 epidote, for instance, are often found with the same characters in the 

 midst of quartz, or imbedded in limestone or dolomite. This inde- 

 pendence of the silicates in relation to their gangue appears to show 

 that minerals are not the simple products of liquation, since media 

 so different would not have secreted identical combinations. Besides, 

 we meet everywhere in metamorphic rocks with minerals very un- 

 equally fusible, which have crystallized in a succession entirely op- 

 posed to the order of their fusibility. 



Arguments of various natures, therefore, oppose themselves to the 

 admission that a metamorphism having no other cause than heat could 

 have given rise to the minerals that we find in the rocks which have 

 undergone its action, even when they do not appear to contain simple 

 bodies foreign to their primitive normal condition. But how much 

 more is this conclusion to be relied upon Avhen we see, as in Brazil, 

 the altered condition of the rocks visibly coincide with the introduc- 

 tion of peculiar bodies, which, according to all probability, could not 

 have come there until after their formation. 



CHAPTER II. 



OF CERTAIN VAPORS CONSIDEREB AS AUXILIARIES TO HEAT ; THEIR ACTION 

 COMBINED WITH HEAT IS STILL INSUFFICIENT. 



If heat alone could not produce the effects of which we have just 

 epoken, will its action, aided by certain bodies, either gaseous or 



^ The association of graphite, with the silicates having protoxide of iron for a base, as 

 mica and amphibole, could not have taUen place at a higli temperature, (Bischofs Gedogie, 

 vol. ii, p. 60,) for at the least a partial reduction of iron to a metallic state would have 

 resulted. The frequent presence of graphite in limestone led Bischof to the same con- 

 clusion, for the two bodies at a high temperature would react the one on the other. 



