62 OBIGIN- AND DEVELOPMENT OF ORE-DEPOSITS, 



were intruded, must equally have been charged more or less 

 directly from below, as will be shewn immediately. 



4. The somewhat different relations of copper to the erup- 

 tive rocks as compared with tin ; and the very different relations of 

 lead, antimony, and other metals, have already been referred to. 

 These metals, except in the shallower portions of veins, where 

 they are in most cases obviously of secondary formation, occur 

 generally as sulphides. The deep-seated source of the sulphide 

 minerals has been already referred to, but it has been urged that 

 sulphides of iron, copper, zinc, lead, and antimony could not 

 exist at the high temperature requisite for the fusion or pseudo- 

 fusion of such rocks.* and hence that subsequent impregnation 

 or mineralization is indicated whenever such sulphides are 

 observed in eruptive rocks. But although such a subsequent 

 impregnation is possible enough, and likely enough in many 

 instances, there are others in which the original presence of 

 sulphides as rock-constituents is plainly indicated. In any case, 

 the argument seems to have but little force, for while under 

 great pressure, and in the absence of free oxygen, it is hard to 

 see how sulphides could be decomposed, if they already existed 

 as constituents of the semi-fused masses beneath the circulation 

 zone. In fact, the existence of copper and iron sulphides as 

 evident original constituents of eruptive rocks has been estab- 

 lished by direct observation in many countries, and by several 

 observers.! 



5. Though the formation and change of our mineral 

 deposits cannot be said to have entirely ceased even now, the 

 most active periods of interaction terminated long ages ago, 

 when what is now the surface, was covered by thousands of feet 

 of rock which have since been removed by denudation. It is 

 generally admitted that the granitic rocks were forced into or 



*" It is well-known that iron-pyrites cannot exist at the temperature of fused 

 granite." R. Pearce, " Influence of lodes on rocks," Rep. Min. Assoc, of Corn, 

 and Devon, 1864. 



f The eruptive-garnet rocks of New Mexico, Arizona, and Chihuahua, contain 

 chalcopyrite in some cases as a sort of base in which garnet-crystals have been 

 formed, and in other cases the garnets enclose " shots " of chalcopyrite just as 

 shots of copper are found in a copper-matt. 



