2 ECONOMIC GEOLOGY 



the igneous rocks and the subsequent extraction, segregation and 

 concentration of the ore materials through weathering processes. 

 (3) Production of metalliferous bodies in connection with rock 

 masses in a molten state, either through magmatic segregation or 

 by the expulsion of the volatile compounds of the metals during 

 the process of magma-cooling. (4) Derivation of metallic par- 

 ticles from extra-terrestrial sources, and their later segregation 

 through the action of surface water. 



Many early writers, including the eminent chemist Bischof, 

 argued that sea water was the primitive source of the metallic 

 salts in nature. They claimed that the metallic salts of the 

 ocean were gathered into ore bodies where marine sediments were 

 laid down. The idea has its foundation in the erroneous assump- 

 tion that rock masses undergo no change. 

 . The answer th'en to the query, what is the primary source of 

 the ores, is the igneous rocks: Either the deep-seated masses upon 

 which the earliest sedimentaries were deposited, or intrusives 

 brought into the sedimentaries in a plastic or fluid condition. 



A certain amount of volcanic water is intimately associated 

 with the development and enrichment of some ore deposits 

 Even granites at the time of their formation contain much water, 

 which is liberated upon cooling, or when they are brought near 

 the surface by faulting or erosion. The general effect of mag- 

 matic waters is reserved for discussion in the chapter on The 

 Origin of Ore Deposits. 



Third, the source from which the water derives the metals is 

 the zone of fracture. The jointing of limestones, granites, and 

 basalt is familiar. It must be along that line of fracture that the 

 water seeps, yet there is a considerable amount of water filling 

 the interstices of the rocks themselves. The water must gather 

 up the material for transportation along these lines and deposit 

 it again thus forming an ore body. 



Fourth, the force which drives the water in its circulation is 

 gravity. This is the recognized force that raises the water in the 

 artesian well, that drives natural gas to the dome where it is found, 

 and petroleum to the pool where it is collected. 



Three courses have been advanced for the trend of solutions. 

 (1) The theory of the descensionists. (2) The theory of the 

 asscensionists. (3) The theory of the lateral secretionists. 



The first theory that meteoric waters bring the minerals into 

 solution and carry them to the lower depths has an extremely 



