IMMENSE SALT CONCRETIONS 191 



garding 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 Carbon- 

 iferous 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, Illi- 

 nois. 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. 



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