To pay railroad charges on ores carrying perhaps 80 to 90 per cent. of useless material would be prohibitive.
The waters which deposit vein stones and ores are commonly hot, and in many cases they have derived their heat from intrusions of igneous rock still uncooled within the crust.
The steaming water rises through fissures in volcanic rocks and is now depositing in the rifts a vein stone of quartz, with metallic ores of iron, mercury, lead, and other metals.
Hence the iron and steel mills east of the Alleghanies--compelled to rely on limited local deposits of Bessemer ore, and upon foreign ores which were constantly rising in value--began to sustain a serious competition with Western mills, even in Eastern markets.
Many attempts had been made in by-gone days to concentrate the iron in such ores by water processes, but with only a partial degree of success. The impossibility of obtaining a uniform concentrate was a most serious objection, had there not indeed been other difficulties which rendered this method commercially impracticable.
Many of the richest ore deposits are thus due to successive concentrations: the ores were leached originally from the rocks to a large extent by laterally seeping waters; they were concentrated in the ore deposits of the vein chiefly by ascending currents; they have been reconcentrated by descending waters in the way just mentioned.