Emily Dickinson Lexicon

ROCK'-SALT, n.

Fossil or mineral salt; salt dug from the earth; chlorid of sodium. But in America, this name is sometimes given to salt that comes in large crystals from the West Indies, which salt is formed by evaporation fro sea water, in large basins or cavities, on the isles. Hexahedral rock-salt occurs foliated and fibrous. – Ure.