236 M. Amedee Burat on the Variations of 



and other exterior causes, so that these variations ought not 

 to be ascribed to posterior alterations, but rather to the ge- 

 nerating phenomena themselves. 



We have no desu*e to extend these conclusions in an ab- 

 solute manner, by pretending that all the phenomena attri- 

 buted to spontaneous alteration are erroneous. Even among 

 the examples on which we have founded our observations, 

 the vein of Rheinbreitenbach presents us with many circum- 

 stances of alterations and molecular transportations. We 

 find there a cross vein filled with steatitic and basaltic de- 

 bris, penetrated at its contact with the vein with native cop- 

 per, which lines its fissures to a distance of many yards. This 

 fact, as well as many other facts of detail, may well be the 

 result of a posterior action. But the great variation of which 

 we have noticed the principal examples, differ completely 

 from tliese details, and are out of proportion with the causes 

 to which they are ascribed. 



The facts here brought forward and discussed, may help 

 further to give us more correct notions of the general theory 

 of metalliferous veins. The shew us the results of subter- 

 ranean emanations, becoming modified more and more as we 

 remove from the seat of the generative actions, so as to form, 

 according to Elie de Beaumont's observations,* zones of a 

 diff'erent nature. 



Thus the sulphuretted or oxidulated, and perhaps native 

 minerals, form the lowest zone with which w^e are acquainted, 

 which sometimes shews itself at the surface, under the form 

 of eruptive repositories ; to these minerals we assign as a 

 character a compact state and homogeneousness in the mass. 

 We find these characters very prominently marked in the 

 eruptive oxidulated iron of the Island of Elba, and in that of 

 Taberg, in Sweden : we find them in the pyrites and other 

 minerals contained in the amphibolites of Tuscany and Nor- 

 way. The native metals contained in the traps are always 

 in a compact state, and are, in this respect, distinguished 



^ Geological Society, meeting of 3d July 1847. 



