A detailed log of initial susceptibility, based on 6135 measurements made on drill core with a hand held meter, is presented for the 1919-m-long lower part of the IRDP Reydarfjordur crustal section. The log shows typical strong internal variation, with a central maximum, in flow susceptibility while dikes show little internal variation. Differences between dikes and flows and internal variations are explained in terms of variation in degree of deuteric and hydrothermal alteration, grain size, and abundances of iron-titanium oxides. General decrease of flow susceptibility with depth is related to depth variation in hydrothermal alteration. This has the effect of removing iron from oxides probably for incorporation in nonmagnetic silicates. This effect is intense toward the bottom of the drill core. Linear extrapolation of the decrease of flow susceptibility with depth indicates a zero value at about 4 km beneath the original surface of the crustal section. Whether or not magnetization of the crust as a whole reaches zero level will depend on the dike at this depth.