Metaviorphic CJianges of Rods, S^c. 117 



stratified primordial rock presents in the different portions of its 

 substance, and he inquires if it be rational to admit that the 

 liquid compound which deposited this rock had every instant 

 changed its composition so as to vary its deposit, and if it be 

 not much more simple to admit that some external and variable 

 influence has produced this change upon deposits which were 

 originally made in a manner which was wholly homogeneous. 

 Besides these local observations which demonstrate this influ- 

 ence, there is a more general one which more decidedly esta- 

 blishes the matter ; it is, that the alteration is both greatest and 

 most varied in proportion as the modified slate approximates 

 to the erupted rocks. 



The author enumerates four modes in which the original ar- 

 gillaceous slate is altered, which, affected by the influence of the 

 burning, and especially the liquid rocks of eruption, have trans- 

 formed it into products which, by their appearance and compo- 

 sition, are very different among themselves, and not less so from 

 the original argillaceous slate. Calcination is one of these 

 modes. Thus, in contact with that trap and hornblendic rock, 

 which he denominates minette,M.Fournet has observed that the 

 argillaceous slates are baked into porcelain jasper {thermantide). 

 This operation, though not quite so simple as he states, many 

 circumstances requiring notice before the result can be accu- 

 rately traced, appears nevertheless to be quite true. The se- 

 cond mode is what M. Fournet calls bmzure, or the method of 

 trituration. This is principally seen in the transition forma- 

 tions. The siliceous argillaceous rocks which enter into their 

 composition have been first broken, and, as it were, triturated by 

 the elevation of the plutonic rocks ; then these fragments, sur- 

 rounded by the fusing matter, have had their angles and edges 

 blunted by commencing fusion, and after this they have been 

 finally soldered by this same cement. The rock which is thus 

 produced exhibits itself under the form of a siliceous trap 

 tuff, a kind of breccia so common in the transition formations. 

 An example of this change may be seen in the Mouette vein 

 near Chessy. 



Other methods become more difficult of comprehension, and 

 consequently of admission, such as those by which the argilla- 

 ceous slate has been changed by fusion and subsequent crystal- 



