SOURCE OF THE. METALS. 1071 



time the ores were deposited as the dominant metasomatic minerals, the ores 

 were probably formed in the zone of anamorphism or rock no wage, prob- 

 ably through the influence of gaseous solutions. The ore deposits which 

 show these minerals as accompanying metasomatic products are few in 

 number and unimportant in amount as compared with the ore deposits in 

 which the metasomatic minerals are those of the first group mentioned. 

 The point to be enforced at present is that the great mass of metasomatic 

 minerals formed at the time the ore deposits are produced shows that 

 these ores were deposited within the zone of fracture, or the zone of 

 katamorphism. 



This conclusion is confirmed by the close relations of the gangue 

 minerals to the country rock. Where a vein runs through a quartzite or 

 very siliceous rock, the gangue mineral is likely to be dominantly quartz. 

 Where a vein runs through basic igneous rocks, the gangue minerals are 

 likely to be largely zeolites and other hydrous silicates, carbonates, and 

 quartz, all decomposition products of such rocks. Where the veins run 

 through limestone the gangue material is likely to be mainly carbonate. 

 It is therefore clear that the gangue minerals are largely segregated from 

 the immediately adjacent rocks traversed by the veins. If this be so for 

 the dominant minerals, there is every reason to believe that the same con- 

 clusion applies to the very subordinate amounts of valuable metals which 

 constitute a small or almost an inappreciable part of the filling of an ore 

 deposit. Numerous examples could be given illustrative of this principle. 

 One of the best known to me is that of the Lake vein in the San Juan 

 district. This vein, running through the volcanic breccias, extended into 

 the limestone below, and from this into sandstone still lower down. Where 

 the vein was in the breccia it was a quartzose vein carrying metals; when 

 followed into the limestone it was seen to become a calcite vein, and when 

 followed into the sandstone it gradually changed again into a quartz vein. 

 In the limestone and sandstone the vein was barren. In this case it seems 

 little short of certain that the vein filling was dominantly derived from the 

 adjacent rock, and there is no reason to doubt that in the San Juan tuff the 

 metalliferous material and the quartz alike were derived from the breccias. 



While it is held that the waters derive the metals, as their immediate 

 source, from the zone of fracture, it does not follow that the metal for the 

 ores deposited from aqueous solutions may not have been derived from 



