Scanning electron microscope and Scanning transmission electron microscope observations of thin sections and separates of the Devonian Onondaga and Helderberg limestones and Ordovician Trenton limestone in New York state allow us to identify three types of magnetite: pseudoframboids, nonspherical magnetite, and fine‐grained magnetite. Magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes. Fine‐grained magnetite, consisting of aggregates of one or more rounded single crystals, approximately 2000 Å (200 run) in diameter, could not be observed in thin sections of New York carbonates due to its small size but was found in magnetic extracts. Hysteresis measurements of magnetic extracts verify that fine‐grained magnetite is capable of carrying remanent magnetizations. However, pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain‐sized crystals that also can be carriers of the remagnetization. These data, in combination with observations from other localities, collectively imply that the Alleghenian remagnetization is carried by the fine‐grained magnetite, although pseudoframboidal and nonspherical magnetite may also contribute. Thus the Alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite. Crystallization of magnetite was mediated by fluids, with dissolution and crystallization activated by stress during the Alleghenian Orogeny. The origin of such fluids is unknown, but they may have originated through crustal scale fluid migration. Tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of