Normal marine deposits represent two independent but associated phenomena, clastic deposition and chemical salt precipitation. Widespread and evenly bedded marine clastic sediments owe their character to a combination of two factors: near-shore deposition coupled with shore migration. With regard to salt precipitation the salinity of the sea and various rivers is considered in connection with known saturation constants for different mineral salts. It is found that for any large portion of the sea to become saturated with sodium chloride, through evaporation, it is necessary that the enclosing basin become completely isolated from the open sea, also a condition probably essential to the formation of gypsum or anhydrite, but not essential to the precipitation of calcium car onate. It therefore appears that the large marine salt basins of the geologic past must have been border basins, filled by sea transgression and isolated by sea retreat. Thus the salt should make up the middle series of sediments deposited during that marine cycle.

No large basin can be isolated from land drainage. Analysis of the effects of streams flowing from surrounding lands into desiccating marine basins shows that the normal precipitated rock sequence produced, as effected by shore recession under the influence of evaporation, is, from the bottom upward, limestone, gypsum and/or anhydrite, rock salt, anhydrite and/or gypsum, and limestone. The first three illustrate Usiglio's sequence; the latter two indicate the origin of the "cap rock," and provide one means whereby the age of the salt may be determined.

The dips of the strata of the Gulf Coastal Plain, generally south or southeast, decrease progressively up through the younger horizons. This is shown to be the normal result of repeated marine invasions. Salt occurrence is not uniform throughout the Coastal Plain. It is found at Smackover, in the Texas and Louisiana interior areas, and in the Gulf Coast proper. Several characters indicate the salt to be of different ages in the several areas of known occurrence, whence it follows that these different areas must represent different basins separated and bounded by pre-salt structures now deeply buried. These structures include the initial Sabine arch, with other structures south of Smackover and south of the Texas-Louisiana interior areas, respectively. Analysis of the overlying sedimen s shows all of these to be of pre-Comanche age.

The Mesozoic strata of the Gulf Border begin with the earliest Comanche, and they rest on a floor of Pennsylvanian or older rocks disrupted by the Ouachita and Appalachian orogenies. No intervening Permian, Triassic, or Jurassic strata are known.

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The lack of these as border facies precludes the possibility of a Permian, Triassic, or Jurassic age for the salt, and analysis of the Comanche sediments indicates the Smackover salt to be lowest Lower Comanche and the Texas-Louisiana interior salt to be Glen Rose.

On faunal, paleogeographic, climatic, and other bases the Gulf Coastal salt is indicated to be middle to late Upper Cretaceous. In this position it represents the Danian stage of the European section, which elsewhere in North America is lacking in a marine equivalent.