THE ORIGIN AND DESCENT OF ROCKS. 475 



of the earth, these waters being supposed to be especially mineralized. 

 Direct evidence on this point is obviously beyond reach. Segregation 

 in the molten state is recognized as a source of ores, but its function is 

 probably confined chiefly to partial enrichment as stated below. There 

 are other occasional methods, but the chief process of concentration, 

 immeasurably surpassing all others, consists in the leaching out of ore 

 materials disseminated through the country rock and their redeposi- 

 tion in segregated forms, as an incident of the recognized system of 

 water circulation. 



Original distribution. — The original distribution of ore material 

 through the primitive rocks is beyond the ken of present science, for 

 even the nature of the true primitive rocks is unknown. For present 

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

 as either igneous or sedimentary, and to inquire, as a first step, how far 

 ordinary igneous and sedimentary processes contribute to the segre- 

 gation of ore material, leaving for a second stage of inquiry the sub- 

 sequent processes of concentration. 



Magmatic segregation. — In a few instances workable masses of ore 

 seem to have arisen from lavas by direct segregation in the molten state, 

 without the aid of subsequent concentration by water action, on which 

 most ores are dependent. It is not improbable that the segregation 

 of metafile iron and nickel, and perhaps other metals, in the deeper parts 

 of the earth may be a prevalent process, giving rise to masses fike the 

 native iron found in basalt in Greenland. This iron closely resembles 

 the nickel-irons of meteorites, which may be illustrations of similar 

 action in small planetary bodies that have been disrupted. Metafile 

 masses so segregated presumably gravitate toward the planetary cen- 

 ter and hence, whatever their inherent interest, have little relation to 

 a subject whose basal criterion is economic. It is not at all -improb- 

 able, however, that in the magmatic differentiation of the lavas that 

 come to the surface, there is some metallic segregation that may make 

 the enriched parts effective ground for the concentrating processes of 

 water circulation, and so determine the location of ore-deposits. Igneous 

 rocks are not equally the seats of ore-deposits, even when the circulatory 

 conditions seem to be equally favorable. These conditions may not 

 really be equally favorable, but there is good ground to befieve that some 

 igneous masses constitute a richer field for concentration than others. 

 No definite rule, however, for distinguishing rich varieties of rock from 



