The interface between peat and underlying limestone bedrock in the Northern Florida Everglades is often occupied by an unlithified calcitic seat earth that is stratigraphically similar to coal-basin seat rock and contains a complex mineral assemblage composed of 14 different minerals. The distribution and characteristics of the clay-size fraction were used to indicate sediment-source areas, depositional environment, and the degree of alteration of the original mineral assemblage. X-ray-diffraction analysis of 42 samples showed that a significant lateral variation exists in the clay-mineral suite of seat earth with respect to location in the study area. The clay minerals palygorskite, sepiolite, and illite were found primarily towards the center of the study area and were derived from sediments north of the Everglades. Seat earth located along the basin flanks showed an increase in the abundance of mixed-layer smectite, kaolinite, and chlorite. The proposed source area for these minerals is from erosion of marginal marine-bar deposits that rim the basin. Underlying limestone bedrock erosion contributed to the detrital mineral assemblage but was not as important as source areas outside the basin. The study-area seat earth is interpreted to have been deposited in an alkaline environment that allowed much of the original detrital mineral assemblage to be preserved. The only exception to this was the occasional identification of well-crystallized, authigenic mixed-layer chlorite-smectite that occurred in samples located near the basin flanks, which were presumably deposited under more acidic conditions.