Geodes are abundant in the lower member of the Warsaw Formation in its type area. Early diagenetic calcite concretions were transformed into geodes as follows: A shell of chalcedony replaced calcite at the rim of the concretion; chalcedony spherulites and quartz euhedra replaced calcite inside the concretion; acidic solution formed a cavity in the recrystallized calcite core; and crystals of several minerals were deposited in the cavity. Specimens in all stages are found together at some places. Concretions grew during early diagenesis in calcilutites (later dolomitized), but not in the associated biocalcarenites. Products of anaerobic decay raised the pH locally causing calcite to precipitate in the concretionary form.