Hysteresis measurements, Lowrie-Fuller testes, Cisowski test, and low-temperature demagnetization experiments on samples of the Knox Dolomite, Trenton Limestone, and Onondaga Limestone indicate that the remanence in these remagnetized carbonate units resides principally in single-domain magnetite, in contrast to previous interpretations. Volumetrically, however, a significant fraction of the magnetite is coarser grained, as previous scanning electron microscopic work on magnetic extracts has shown. These conclusions are based mainly on the characteristic ''wasp-waisted'' shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity, both of which clearly indicate a bimodal distribution of coercivities/grain sizes. The magnetization is controlled mainly by cubic magnetocrystalline anisotropy, consistent with the spheroidal particles, commonly observed in extracts. Hysteresis trends show that especially the finest particles lack shape anisotropy. In view of these conclusions, the two components of natural remenance in these rocks are interpreted as (1) a late Paleozoic (Kiaman) chemical remanence carried by single-domain diagenetic magnetite and (2) a Cenozoic thermoviscous overprint residing principally in coarser, pseudosingle domain and multidomain magnetite, also of late Paleozoic diagenetic orgin. ¿ American Geophysical Union 1990