150 Proceedings of the Royal Irish Academy. 



rock, -we sIlouIcI be entitled to consider its presence accidental ; bnt 

 its occurrence in small amount, and its being generally diffused in the 

 rock or mineral, proves it to be truly a constituent. 



A lode or thick deposit of zinc ore Tvould be an accidental de- 

 posit ; but it is from the infinitesimal quantities of this metal dis- 

 seminated throughout rocks that workable accumulations are derived. 

 As Bischof remarks, the minimum of a mineral in rocks becomes the 

 maximum in lodes ; and, although the small traces of zinc in the 

 specimens given above may appear insignificant, it must be remembered 

 that a knovrledge of the fact of the diffusion of minute quantities of 

 the metallic compounds thi'ough rocks leads to a correct notion of the 

 formation of mineral veins, as otherwise we should be compelled to 

 regard them as exotic productions, derived fi'om unknown sources. 



