The Mifflin Member of the Platteville Limestone Formation changes from limestone to dolomite as it is traced eastward from the type locality south of Platteville, Wisconsin. The presence of interference ripple marks, scour and fill, and rip-up structures indicated that the Mifflin was deposited in a very shallow epicontinental sea. The study reveals that petrographic analysis can be used to subdivide the Mifflin into three facies; a biomicrite facies on the west, a biosparite facies in the middle, and a dolomitized biomicrite facies on the east. The distribution of sparite cement indicative of a mature, high energy limestone is restricted to a narrow zone situated between the limestone and dolomite facies. This narrow zone is interpreted as a biosparite carbonate shoal or bar that partially cut off the dolomite facies from the open water limestone facies. Dolomite is restricted to the crest of the structural high known as the Wisconsin arch (fig. 1). The presence of this structure at the time of deposition of the Mifflin caused the formation of a carbonate shoal or bar behind which was a semi-restricted shallow lagoon where conditions were favorable for dolomitization. However, the limestone on the flanks of this structure outside the lagoon was not dolomitized. This study also reveals that the micrite matrix was selectively dolomitized as the result of its finer grain size, greater surface area, and the possible presence of large amounts of meta-stable aragonite and high Mg (super 2+) calcite at the time of deposition. The intraclasts (rip-ups) of sucrose dolomite found incorporated into the limestones show that marine dolomitization was pre-deep burial or early diagenetic. The few microcrystalline dolomite clasts and fossil fillings incorporated in a matrix of sucrose dolomite are interpreted as penecontemporaneous. It is concluded that there were two stages of marine dolomitization, an early diagenetic and a penecontemporaneous, the most extensive being the early diagenetic. In addition to these two stages of marine dolomitization there is evidence of subsequent dolomitization due to secondary ground water circulation (Deininger, 1957, p. 281) and hydrothermal dolomitization associated with lead-zinc mineralization (Agnew, 1959, p. 96) in the southwestern part of the area studied.