Fine-grained lacustrine silts from 25 to 75 feet thick cover hundreds of square miles on the floor of glacial Lake Agassiz. In numerous localities an enriched dolomitic zone from 1 to 10 feet thick occurs in the silts at from 1 to 3 feet below the surface. The modes of occurrence of the enriched zones indicate that the dolomitic bodies are not simply a product of negative enrichment formed by the leaching of calcium carbonate. Most of the dolomitization has taken place where the water table is near the surface or where the natural drainage is poor. The ground waters contain a high percentage of sodium and of magnesium salts. Gypsum nodules and crystals are found with and below the dolomite. This association suggests a replacement of calcium by magnesium and the precipitation of calcium as gypsum.