Page:Popular Science Monthly Volume 82.djvu/195

Rh the source of the brines that have fed these growing crystalline masses. It is well understood that the thick coal-bearing rocks of west central Arkansas derived their material from the south. The Carboniferous continent extended Gulf-wards doubtless as far as the southern limits of Louisiana and perhaps considerably beyond. These old lands were eroded and swept northward into the Carboniferous seas of west Arkansas already referred to. In Permian or slightly later times this continental area was base-leveled, standing on a par with west Kansas and north Texas, receiving deposits of salt and gypsum in shallow sun-baked seas. After considerable accumulation of these saline materials the Gulf region was depressed at the south and covered by later and later deposits and the Gulf invaded the Mississippi valley to Cairo, Illinois. This was in late medieval geological times (late Cretaceous). Since then the central part of the continent has gradually raised, the Gulf border has sunk so that, through the Tertiaries and recent ages, the formations have been tilted more and more to the south till the salt-bearing Permian beds are doubtless 5,000 to 8,000 feet beneath these younger deposits. Hence in all probability the source of the artesian flow of brines that produce the salt masses under discussion.