PROCEEDINGS: GEOLOGICAL SOCIETY 29I 



pounds to the square inch. Where the water rests upon it the pressure 

 is only about half as great. Salt being somewhat plastic the difference 

 in pressure would no doubt cause the sand grains to sink into the 

 salt and the salt to rise between sand grains until friction put an end to 

 the adjusting process, which might be in a fraction of an inch. 



Now suppose a rising igneous plug approaches the salt bed and raises 

 the temperature until, with whatever aid may be rendered by the 

 ever-present water, it melts the salt. The process outlined above 

 would then proceed to a much greater extent, the salt rising like so 

 much water until the supply near enough the plug to be melted was 

 exhausted. Apparently the process might involve the pushing aside 

 of a large body of sand and other materials and the development of a 

 considerable mass of salt, accompanied by unusual secondary deposits 

 due to the unusually high temperatures. 



G. S. Rogers: Origin of the salt domes of the Gulf Coast. The origin 

 of the great plugs of salt that occur here and there beneath the Coastal 

 Plain of Texas and Louisiana is an obscure problem which has been a 

 subject of much speculation. The small diameter of these plugs and 

 their great depth (at least 5,400 feet in once case) indicate that they are 

 not original bedded deposits. Nearly all x\merican writers have adopted 

 the view that the salt has been deposited by ascending brines, but the 

 plausibility of this theory is injured by the stupendous quantitv of 

 brine involved, for no likely condition has been suggested under which 

 the waters would deposit more than a fraction of their dissolved load. 



Although direct evidence of the mode of formation of the salt plugs 

 is scanty, there are several indications that positive tectonic forces have 

 been involved. The plugs are arranged rather regularly along lines 

 that are undoubtedly related to the main structural features of the 

 region. Moreover, the plugs have caused a sharp and very local up- 

 thrust or doming of the normally flat-lying sediments, the uplift in 

 some cases amounting to at least 3000 feet. The surface beds may be 

 only slightly disturbed, if at all, but down on the flanks of the salt mass 

 dips of 60° or more are common and in some cases formations have 

 been thrust clear through the beds that normally overlie them. Still 

 more direct evidence is afforded by the structure of the salt itself; 

 as seen in the mines the salt contains dark streaks which, though com- 

 monly standing almost vertical, are in places thrown into intricate 

 folds which bear an extraordinary resemblance to the flow structure of 

 ancient rocks. The common elongation of the salt crystals in a ver- 

 tical direction and the rough horizontal cleavage of the salt also suggest 

 vertical movement. 



Laboratory experiments have shown that salt under differential 

 pressure behaves as a highly plastic substance; that its plasticity is 

 increased by heat; and that if shattered it is easily welded by pressure. 

 In view of the field evidence cited, the writer believes that the salt 

 plugs are offshoots of deeply buried bedded deposits which have been 

 subjected to great pressure or thrust, and have been partially squeezed 

 upward in a semiplastic condition along lines of weakness. As the 



