208 Proceedings of the Royal Physical Society. 



B 2. The Original Contents of Mineral Veins. 



Mineral veins, such as those of Wanlockhead and Leadhills, 

 containing a large number of mineral species, are probably of 

 considerable geological antiquity ; while others, containing a 

 small number of species, may well be comparatively recent in 

 oiigin. If these veins are regarded in a very broad and general 

 way, they appear to be referable to two great classes : — (1) 

 those whose contents have arisen from below and have ever 

 since remained nearly in their original condition ; and (2) those 

 whose constituents, in whole or in part, have formerly been 

 at a higher level than they are at present. It is with those 

 belonging to the first category that we have here to do. 



It has long appeared to me that the only satisfactory 

 explanation of the origin of the normal type of mineral veins 

 is that it represents a series of deposits from thermal waters 

 which have risen from the heated interior of the Earth's 

 crust in the direction of the surface, and have deposited the 

 substances they originally held in solution when they started, 

 one after another, on the walls of the cavity, in accordance 

 with the temperature which determines the depositing point 

 for each species. The great majority of mineral veins either 

 occupy fault-fissures or else are closely connected with these 

 zones of communication between the interior and the exterior 

 of the Earth. It is assumed that thermal springs are con- 

 nected directly or indirectly with volcanic action — usually 

 they appear to mark the waning of a volcano. It is highly 

 probable that the number of mineral substances held in 

 solution by the thermal waters is proportionate to the eleva- 

 tion of temperature. As the waters rise in the direction 

 of the surface, the depositing temperature of one mineral 

 substance after another is successively reached, until, by 

 the time the spring reaches the surface, very few of the 

 numerous substances of which it was formerly the vehicle 

 still remain in solution. 



It should be noted that the zone where any given sub- 

 stance is deposited from solution varies with the thermal 

 conductivity of the rock it is traversing, and, also, with the 

 lowering of temperature consequent upon the deflux of 



