Metallifermts Veins. 173 



but the case is different with respect to veins which are con- 

 nected by mineral transitions with the contiguous rocks. In 

 the latter we are obliged to suppose, that the formation of the 

 rock, that of the vein and its filling, are almost contemporane- 

 ous. On the other hand, when we see the crystalline masses of 

 different substances in the middle of, and on all sides entirely 

 surrounded by rocks equally crystalline, so that we cannot be- 

 lieve they have been introduced into the cavities they fill, either 

 from above or beneath, we are compelled to consider the vein as 

 an open fissure in a crystalline rock, that has been again pene- 

 trated by the substance in solution, which has thus been intro- 

 duced into the cavity and precipitated there, the various parts of 

 the rock being more or less dense, or the solution being more or 

 less saturated in different situations. There is another mode of 

 filling, which has produced the veins which contain metallic 

 sulphurets, in crystalline groups, projecting in all directions in 

 the vein, and bodies which decompose in aqueous solutions, 

 such as the metallic sulphurets and arseniurets, which will not 

 sustain an elevated temperature without decomposition, unless 

 under the action of a considerable compressing force. 



It therefore appears almost certain, that veins have not all 

 been produced by one general cause alone, and tliat many in- 

 fluences have sometimes concurred in their formation. 



It follows, from the brief sketch which we have just pre- 

 sented of the state of our knowledge of the constitution of veins, 

 that it is impossible to admit, that the fissures which have at 

 different periods opened in the rocks have been filled by sub- 

 stances transported thither by the action of terrestrial electric 

 currents, as they exert no chemical action except where solid 

 conducting bodies and liquids, capable of reacting one on the 

 other, exist. Now the rocks are not conductors of electricity, 

 and the solid metallic ores were not in existence when these 

 fissures were formed. We must therefore admit, that other 

 causes than electric currents have filled these rents. The filling 

 up once effected, either entirely or in part, and the water en- 

 tering from the surrounding rocks, electric forces would then 

 intervene to effect decompositions, and give birth to new com- 

 binations. 



