A transition between limestone and dolomite in the MacGregor Member of the Platteville Formation ranges up to a mile in width and is marked by irregular and patchy dolomitization. The transition boundary is traced through Lafayette County, Iowa, and Dane County, Wis. Locally the dolomitized patches roughly parallel the dominant joint set. There is some evidence of permeability control of dolomitization. Source of small amounts of epigenetic sulfides may be related to ore mineralization in the nearby lead-zinc district. Diagenetic history of the rock suggests that either ground water or hydrothermal solutions were the source of Mg for dolomitization.