VEINSTONES. 585 



Werner has informed us that, in many veins, 

 the rock on both sides, or, in the miners' lan- 

 guage, the roof and the sole, the hanger and the 

 leger, is altered and decomposed. This accident 

 chiefly takes place in mountains of granitel, gneiss, 

 mica slate, common slate, and porphyry. But 

 this decomposition seldom extends to more than 

 one of the constituent elements of the rock ; for 

 the quartz remains entire ; while commonly the 

 felspar, often the mica, and very often the horn- 

 blende, are decayed ; the potash of the one, and 

 the iron of the others, being very liable to decom- 

 position. This alteration sometimes extends a 

 considerable way, even a fathom ; and is not 

 always apparent along the vein, but chiefly in 

 those parts where the mineral abounds with sul- 

 phur. In the pursuit of a barren vein, when this 

 decomposition begins to appear, it may be con- 

 cluded that ore is not far distant. 



This change Werner ascribes to acids in the 

 dissolution that formed the vein; and supposes 

 that the felspar is changed into kaolin, or white 

 clay, by the carbonic acid ; and he gives examples 

 of gneiss and granite thus decomposed. He also 

 supposes that the sulphuric acid may affect the 

 mica and hornblende, and convert them into that 

 green bole or lithomarga, which was originally 



