42 GROUND-WATER 



able, lean bodies of ore material. The metal need not preponderate, 

 or form any fixed percentage of the whole. Little gold ore contains 

 more than a very small fraction of one per cent of the precious metal, 

 while high-grade iron ore yields sixty-odd per cent of the metal. 

 In iron ore, the metallic oxide or carbonate makes up nearly the 

 whole rock; in gold ore, the metal is one of the least abundant 

 constituents. 



Metals are disseminated widely through the rock substance of 

 the earth, and even through the hydrosphere; but in their dis- 

 seminated condition they are not ores. The concentration of the 

 metals into workable richness, in accessible places, is the essential 

 thing in the formation of ores. The degree of concentration re- 

 quired is measured by the value of the metal. The chief points about 

 ores to be considered in connection with ground- water are (i) the 

 original distribution of the metallic materials, (2) their solution by 

 circulating waters (or, rarely, by other means), (3) their transporta- 

 tion in solution to the place of deposit, (4) their precipitation in 

 concentrated form, and (5) perhaps their further concentration and 

 purification by subsequent processes. Ores which originated in 

 volcanic intrusions or from waters derived from lavas (magmatic 

 waters) are mentioned but briefly here. 



Original distribution of ore material. For present purposes 

 it is sufficient to regard all rocks concerned in ore-deposition as 

 either igneous or sedimentary, and to inquire, first, how far ordinary 

 igneous and sedimentary processes contribute to the segregation of 

 ore material; and second, what the subsequent processes of local 

 concentration are. 



Magmatic segregation. The segregation of metals in lava is 

 known as magmatic segregation. In some instances masses of iron 

 ore seem to have originated in this way. It is not improbable that 

 the segregation of metallic iron and nickel, and perhaps other metals, 

 may be common in the deeper parts of the earth, but it is not clear 

 that many known ores originated in this way. It is probable, 

 however, that there may be some segregation of metallic substances 

 in lavas. While this segregation may not be rich enough to make 

 ore, it may determine the places where subsequent concentration 

 takes place, by the help of ground-water. 



Marine segregation and dispersion. In the formation of the 

 sedimentary rocks there was notable metallic enrichment in some 

 places. The ground-waters of the land, after their subterranean 



