Page:Popular Science Monthly Volume 82.djvu/193

Rh masses have come from below and have moved upwards. This is as clearly demonstrated as the fact that the battleship Maine was wrecked from a force without "because the plates were bent inwards." Were the Tertiary and Quaternary beds removed from the flanks of these salt masses we should see a cylinder or perhaps more accurately a truncated cone of salt standing upon mid-Cretaceous rocks towering upwards half a mile or perhaps a mile, though the upper end of the cone might not be over mile across. Some one will say that is certainly similar to the church-spire spur that was lifted out of the crater of Mt. Pelée after its recent destructive eruption. Others will be reminded of Bogoslof Island in Alaskan waters. But here again, in endeavoring to explain the phenomenon there is no need of invoking vulcanicity. For the past ten years we have had exceptional chances to study all these interesting salt masses and are prepared to confidently affirm that the origin of both salt masses and their movements has nothing to do with volcanic action.

The true explanation of the origin, growth and movement of these salt masses seems simple when once we have a clear understanding of certain structural features of the lower Mississippi region. Observe on any geological map that Quaternary and older rocks back to the medieval or Cretaceous beds all slope Gulf-wards at a much greater angle than the surface of the ground makes with the horizontal. In other words, if water should enter a pervious Cretaceous or older bed in Arkansas and follow the same to the latitude of the Gulf border it would find itself several thousand feet below the Gulf level. Such waters would naturally become very warm as compared with water at or near the surface. They would take soluble substances in solution. If a break or point of weakness occurred in the superincumbent beds such hot waters would ascend after the manner of water in an artesian well. If the waters were saturated with salt at a high temperature they would be obliged to part with some of their saline burden as they approached the upper, cooler strata. The amount of salt held in solution by water at various temperatures, it is true, increases not greatly with increased heat; nevertheless, it is appreciable, and in the end the giving up of salt by lowering temperature would produce notable results. Again, though salt masses might tend to accumulate as just outlined at a certain place in the crust of the earth, would not pressure prevent such a growth, and even if growth takes place what would tend to push the salt up bodily say 1,000 feet or more? Here again we need none of Vulcan's aid, for we all know that when once crystallization commences each little crystal will have its growth in spite of almost any resistance. Witness the growth of ice crystals in our water pipes in zero weather. In other words, the force exerted by growing crystals is known to be at least of the same order of magnitude as the crushing