Textbooks of not so long ago typically presented the study of stratiform igneous intrusions as a simple and straightforward exercise in nature's petrology laboratory. The fractional crystallization products of mafic magma were presumed to accumulate sequentially, largely in response to gravity. Those familiar with the literature of the past several years are aware, however, that the textbooks of the next generation will conclude that this model, if not actually wrong, is at best a gross oversimplification. Why this apparent fall from grace of such a seemingly useful concept?