Page:Popular Science Monthly Volume 72.djvu/546

 by oxidation or by solvents, the quantity, time and temperature may all be sufficient to accomplish great results with more tractable ores, but have practically no effect upon these refractory ones. A good example of this again is found on the Mesabi range where the heat of an eruptive rock has so altered a portion of the iron formation for many miles that it has resisted surface solution and concentration, and is a worthless low-grade mixture of rock and magnetic ore still; while away from the influence of the eruptive, have been formed the iron ore deposits which have given to the iron and steel industry of this country the raw material required to make us preeminent in the markets of the world.

Reduced to more simple language and ideas the foregoing remarks amount to a statement that climate, sun, rain, average temperature, topography, depth of soil or surface debris, erosion, glaciation and other common and often unobserved influences and conditions have decided bearing upon the important question of ore formation.

These are the phases of our modern theory that have received little attention hitherto; and are yet of practical value that can hardly be overestimated. We find few bonanzas of high-grade ore in Siberia, Russia, Alaska, British Columbia, Washington or northern Ontario. Our theory tells us why they are not to be expected, and why such enriched ores as are found seldom extend downward to great depths. We turn to regions of milder climate, less glaciation, gentler topography, and we find the rocks altered and softened and oxidized to some depth below the surface. We find that the veins wear "iron hats"; and beneath them we find bonanzas reaching to great depths. We find our best ore shoots on the sunny sides of the mountains, while the veins on the northern shaded sides where the snow lies till mid-summer and the rocks are cold produce no such rich ore. We begin to realize that our theory is based on fact and proved by observation; and that it justifies us in placing confidence in it, and in acting upon it within reasonable limits. And we marvel that facts so simple and of such easy comprehension and yet of such practical value should receive so little attention from the writers on ore deposits.