The concept that rock salt of the Gulf Coast salt plugs is virtually pure halite has been established on surprisingly meager data, and is contrary to what should be expected in the light of what is known of salt deposits in other parts of the world. The composition of Gulf Coast rock salt has direct bearing on the origin of cap rock.

The amount and character of water-insoluble residue has been determined for rock salt from 20 Gulf Coast localities, chiefly in Louisiana, and has been found to be much the same as that of the German occurrences. The residues average between 5 and 10 per cent by weight of the rock salt and consist largely of anhydrite grains, but the following minerals, in the order of abundance, have been found in one or more localities: dolomite, calcite, pyrite, quartz, limonite, hematite, hauerite, sulphur, celestite, marcasite, barite, kaolinite, gypsum, magnesite, danburite, boracite, and a new borate. Inclusions of brine, gas, oil, and sand are not uncommon in the salt plugs of the Gulf Coast. Petrographic analyses of the sand inclusions from Avery Island and Jefferson Island show that they dif er from known sands of the Gulf Coast. Potash salts are present in 10 of the 21 salt plugs studied.

Petrographic analyses are to be made of cap rock from several of the salt domes studied in order to determine whether the distinctive minerals of the residues occur in the cap rock in such relationship as to indicate its formation by accumulation of water-insoluble residue from the salt.