MATERIALS OF THE EARTH 69 



Magmatic segregation. In a few instances workable masses of 

 ore (e. g. iron) seem to have arisen by direct segregation in the liquid 

 lava. It is not improbable that the segregation of metallic iron and 

 nickel, and perhaps other metals may be a prevalent process in the 

 deeper parts of the earth, giving rise to such masses of native iron 

 as are found in basalt in Greenland. But masses of metal so seg- 

 regated presumably gravitate downward, and have little relation 

 to known ore-deposits. It is probable, however, that some segre- 

 gation of metallic substances in the lavas that come to the surface, 

 is a rather important first step. The rocks thus enriched perhaps 

 determine the places where subsequent concentration is to take 

 place. The acid igneous rocks (p. 24) are, on the whole, perhaps 

 less rich in ore materials than the others; but there is no established 

 law. It is probable that all sorts of lavas were richer in some places 

 than in others at the time of their origin. 



Marine segregation and dispersion. In the formation of the 

 sedimentary rocks there was notable metallic enrichment in some 

 cases, and depletion in others. The ground-waters of the land, 

 after their subterranean circuits, carried to the seas various metallic 

 substances in solution. In the main these substances appear to 

 have been widely diffused, and to have been very sparsely deposited 

 through the sediments, for sediments seem to contain less ore ma- 

 terial than igneous rocks. But there are important exceptions 

 to this general rule of sedimentary leanness. 



The iron-ore beds of Clinton age ranging from New York to 

 Alabama, and appearing also in Wisconsin and Nova Scotia, form 

 a stratum in the midst of the ordinary sediments, and contain 

 marine fossils. The great iron-ore beds of Lake Superior also were 

 sedimentary in origin, and so were most other important iron de- 

 posits. It cannot be affirmed, in all cases, however, that the iron 

 deposits were marine. In some cases, the ferruginous material was 

 originally disseminated widely through land rocks, and was con- 

 centrated in the course of the sedimentary processes. In other 

 cases it may have been derived from igneous rock rich in iron, 

 extruded beneath water. In either case it is a sedimentary rock, 

 of the non-clastic variety. 



Limestone appears to have been enriched locally in lead and 



