Page:Popular Science Monthly Volume 72.djvu/545

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The time element enters in a variety of ways into the problem of ore formation by descending circulations. Thus an ore deposit formed in its primary, low grade constitution during earlier geological periods, such as the Cambrian or Huronian, and during all of the subsequent ages exposed to the action of superficial agencies unhampered by subsequent covering of later rocks, has a thousandfold the opportunity for concentration of its ores that is presented by similar rocks and ores formed during later geological epochs, say the Tertiary. This is exemplified by the iron ores of the Mesabi range as contrasted with the glauconitic deposits of New Jersey or Texas. During almost all the ages since the Cambrian the iron ore formation of the Mesabi has been exposed to the weather, covered only for a geological moment during a part of Cretaceous time. The result is the largest and purest deposits of iron ore ever discovered, while rocks of similar composition but much more recent formation exhibit only the initial stages of ore formation.

Another way in which time affects ore deposition is in connection with the rate at which the waters move in a vein. Solutions of a given composition may move so rapidly as to produce but little effect, or may move so slowly that they clog up or retard other active waters after their own power is exhausted. Upon a steep drainage slope or mountain the waters may pass off so rapidly, even below the actual top of the ground, as to exert but little influence, or they may move with just sufficient rapidity to accomplish their maximum of chemical effect.

Our third factor, temperature, is of great importance. In the first place, oxidation, which is but another name for combustion, is greatly accelerated or retarded by slight changes in temperature. Sulphides which remain immersed for centuries in water under a glacier in Alaska would be completely oxidized in a few years exposed to the heat of the sun on a southern slope in Colorado or California. In the next place, the rate of solution depends directly upon temperature, increasing as the temperature rises, and, itself a process of heat consumption, is greatly facilitated by heat from external sources. Thus in warm rocks, in mild climates, upon the sunny side of mountains, there will be the most favorable conditions as regards temperature, for the formation of secondarily enriched ore deposits. The experienced prospector will tell you that it is in precisely these localities that they are found, although he never before heard any explanation for it.

Lastly, the physical structure and solubility of the rocks and ores affect their susceptibility to later enrichment for perfectly obvious reasons. A dense rock is not readily entered by mineralizing solutions. Likewise an insoluble one is not easily replaced and does not afford lodgment for ores. And if the ores themselves are not readily attacked