The contents of this paper record the results of a detailed examination of certain extensive areas of dolomitization in the Carboniferous Limestone of the Midlands. During the period 1912 to 1914 the faunal succession in the Carboniferous Limestone bordering the Leicestershire Coalfield chiefly claimed my attention, but many problems associated with dolomitization arose in connexion with that work and have been discussed in a previous paper. Being thus introduced to a study of metasomatism which yielded interesting results in Leicestershire, I turned my attention to the larger Carboniferous Limestone outcrop in Central Derbyshire, where a considerable thickness of dolomitized beds awaited description. The question as to whether the dolomitization in Derbyshire affected material on similar horizons or exhibited other features analogous to those of the dolomitization of the Leicestershire outcrops afforded considerable scope for investigation. Additional incentive to this particular study lay in the fact that dolomitization in the Carboniferous Limestone of this country had not been treated as a principal theme, but had been briefly described in notes occurring in papers by various authors, whose main object was the elucidation of other problems such as faunal succession.