228 M. Am6d6e Burat on the Variations of 



ample, in which galena is the normal mineral in depth, pre- 

 sent us, throughout the whole superior region, with the car- 

 bonate and phosphate of lead, frequent or even predominat- 

 ing, the sulphate, arseniate, and chloride, being accidental. 

 Silver, so intimately mingled with the normal galena, is in- 

 sulated in the state of native silver, in filaments, branching, 

 or dendritic, sometimes in the state of chloride and bromide. 

 The blende seems likewise transformed, and the zinc appears 

 in the state of carbonate or silicate. 



The cupriferous repositories are those which present the 

 most complex and most striking differences. Thus, while 

 sulphuretted or pyriteous minerals constitute the normal mi- 

 neral in depth, the modified region, which may be called that 

 of the gossan, presents us with native copper, earthy or crys- 

 talline oxides, hydrosilicates, hydrocarbonates, phosphates, 

 arseniates, and chlorides; minerals remarkable for their beau- 

 tiful colours, and which thereby give a peculiar aspect to the 

 repositories. 



The passage of these minerals belonging to the superior 

 regions into the sulphurets, which exclude them at greater 

 depth, does not take place suddenly. There is always a zone^ 

 of mixed characters in which the mineral substances are 

 blended. Another character agrees with that of the change 

 of composition in depth, namely, that of structure. In veins, 

 for example, we know that the ribbon structure, that is to 

 say, parallel zones on the roof and walls, is generally ob- 

 served ; but this ribbon arrangement no longer exists in the 

 altered parts of the superior regions, which have always a 

 massive and fragmentary structure without regularity. In 

 irregular repositories and veins not subject to ribbon forms, 

 although the change of the structure in depth be in some 

 measure indefinable, yet we observe it pretty easily because the 

 sulphuretted and crystalline elements of the deeper parts are 

 almost always conjoined with a predominating system of 

 structure. The metalliferous matters are found either in 

 thick zones or in small veins, in>drusy cavities, nodules, or 

 lenticular and rounded masses, while the modified parts pre- 

 sent nothing but confused masses and irregular impregna- 

 tions. 



