Iodine is present in rocks, from which it is released when the rocks decompose during the process of weathering. It is dissolved in rain water and is carried to the oceans by the rivers. This, no doubt, is one of the reasons why the ocean is so rich in iodine, as well as other minerals which are carried to it in a similar way. While various other salts remain in the sea, iodine evaporates and is returned to the soil by means of rainfall, dew, fog and snow. Every year hundreds of tons of iodine are thus returned to replenish the soil.
In mountain valleys, however, a deficiency of iodine is often found in the population. The explanation for this may lie in the fact that the water rushes along so rapidly that it carries away large amounts of dissolved iodine instead of restoring it to the soil, as it is able to do in the flatlands.
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