290 proceedings: geological society 



assic than in any other group of beds. The copper and other metalHc 

 minerals, small quantities of which have been reported at one or two 

 places, harmonize with the hypothesis. 



The lithologic features of the Cretaceous and Tertiary that are of 

 special interest in the salt-dome problem are the generally more clayey 

 and apparently also more carbonaceous constitution of the material 

 west of the Mississippi as compared with that to the east. Apparently 

 the salt domes occur where the beds are thicker, softer, and more 

 carbonaceous, and the writer suspects that there is some relation be- 

 tween these characteristics of the region and the domes. There are, 

 of course, a good many more or less thoroughly indurated beds including 

 chalk and limestone west of the Mississippi, particularly in the older 

 formations, but the mass of Mesozoic and Cenozoic materials as a whole 

 seems to be of a conspicuously yielding nature. 



The great thickness and the low degree of rigidity are apparently 

 related to a westward littoral current which, though not strong enough 

 to carry sand except along the strand, has for ages carried ver}/ fine- 

 grained and clayey material westward from the mouths of the Missis- 

 sippi and other ancestral and present-day rivers discharging from the 

 central and eastern portion of the Gulf embayment. The most striking 

 general feature of the samples that I collected on the Fish Hawk ex- 

 pedition, a few years ago, is the contrast between the dark and clayey 

 sea-bottom material west of the Mississippi and the lighter-colored and 

 coarser-grained material to the east. 



Since the deposits are mainly unconsolidated and many layers are 

 plastic it seems presumable that in the process of intrusion, differences 

 in specific gravity between intruding and intended materials will control 

 to a more or less notable degree the form of the intrusion. For example 

 it seems quite possible, if not probable, that if a rising mass of salt or 

 one of molten rock comes to a position where the overlying material 

 weighs less per cubic foot than the rising mass, the mass may cease 

 rising and may even "mushroom," whereas if the overburden were 

 heavier the intrusion might continue to rise and might even develop 

 into an extrusion. The clay and incoherent sand of the gulf coast have 

 an average specific gravity including the water in their pores of about 

 two (probably somewhat less than two) and therefore are a little lighter 

 than salt and much lighter than igneous rock. Hence the absence of 

 igneous rock at the surface cannot be taken as an indication that the 

 salt domes have not formed over igneous plugs. 



Another way that differential specific gravity may play a part in the 

 growth of salt domes is based on the fact that the hydrostatic head 

 at any point underground is only about half the weight of overlying 

 deposits. Assume a bed of salt with a perfectly smooth top lying at 

 a depth of 10,000 feet and upon this salt a bed of sand with interstices 

 filled with water which, by way of connecting interstices, forms a 

 continuous column up to the surface of ground water. Where the 

 grains of sand rest upon it the salt supports a pressure about 8000 



