Microfacies analysis is a mature and sophisticated set of procedures for solving a variety of stratigraphic problems. One of the most important of these problems is the identification and definition of surfaces for stratigraphic correlation. There is virtually no end to the list of potential correlative surfaces that can be defined by microfacies analysis, particularly when used in conjunction with available mathematical techniques for the quantification of geologic data. Such procedures are particularly useful in stratigraphic units characterized by complex fades relationships, such as reef-associated paleoenviron-ments. The utility of microfacies analysis in evaluating potential sites for nuclear power plants and other engineered structures is exemplified by studies performed in conjunction with the NORCO-NP-1 nuclear power plant site in northern Puerto Rico. The definition of three microfacies (Globigerinidea, Nummulites cojimarensis, and A mphistegina spp., in ascending order) within the Quebradillas Limestone (Tertiary) at the NORCO-NP-1 nuclear power plant site demonstrated that two surfaces along which important natural gamma ray peaks occur are strati-graphically controlled rather than being controlled by dislocation surfaces, unconformities, jointing, or karstification. This was accomplished by showing that the surfaces defined by the gamma ray peaks are parallel or subparallel to two deposi-tional surfaces, the base and top of the N. cojimarensis microfacies, within the apparently massive carbonates of the Quebradillas Limestone.