Ch.VL] ORIGIN OF MINERAL VEINS. 101 



either granite or greenstone; but as the rocks gradually 

 cooled, the fissures reached greater and greater depths ; 

 and the lighter constituents having been drawn off and 

 exhausted, only the heavier molten silica, mingled with 

 metallic and aqueous vapours, has been left, and with 

 these the last-formed and deepest fissures have been 

 filled. These injections never reached to the surface, 

 probably never beyond the area of heated rocks ; so that 

 there have been no overflows from them, and they have 

 only been exposed by subsequent great upheaval and de- 

 nudation. 



4. Probably the molten matter was injected into the 

 fissures of rocks already greatly heated, and the cooling 

 of these rocks has been prolonged over thousands of years, 

 during which the lodes have been subjected to every 

 degree of heat, from that of fusion to their present 

 normal temperature. During the slow upheaval and 

 denudation of the lodes, they have been subjected to 

 various chemical, hydro-thermal, and aqueous agencies, 

 by which many of their contents have been re- arranged 

 and re-formed, new minerals have been brought in 

 by percolation of water from the surrounding rocks, 

 and possibly some of the original contents have been 

 carried out by mineral springs rising through the lines 

 of fissures which are not completely sealed by the 

 igneous injection, as the contraction of the molten matter 

 in cooling has left cracks and crevices through which 

 water readily passes. 



5. Some of the fissures may have been re- opened since 

 they were raised beyond the reach of molten matter, and 

 the new rent may have been filled by hydro-thermal 

 or aqueous agencies, and may contain alloy with, vein- 



