= Solution
Strong geological <magnetic anomalies> generally occur in <rocks> containing abundant strongly magnetic iron oxides, especially <magnetite> and <titanomagnetite>. Many <mafic rocks>, including <basalt> and <gabbro>, and some <ultramafic rocks> have large signatures; magnetite-rich iron formations can be exceptionally strong, while <serpentinization> can create additional <magnetite>. Actual <magnetic susceptibility> and <remanent magnetization>, rather than the rock name alone, determine the response. Sedimentary and felsic <rocks> are often weaker but have important exceptions.
<Magnetite>, <titanomagnetite> and <maghemite> exhibit strong <ferrimagnetism>; some <pyrrhotite> is also strongly magnetic. <Hematite> is generally much weaker in induced <magnetic susceptibility>, although it can carry stable <remanent magnetization>. <Quartz> and <calcite> have very small, usually negative, susceptibilities, and many ordinary silicates have only weak <paramagnetism>. A steel car exhibits strong <ferromagnetism> and can therefore stand out against a weakly magnetic host, but its shape and pre-existing <remanent magnetization> complicate the signal.<Applications manual for portable magnetometers by Geometrics>[Rock-susceptibility discussion and the section “Remanent or Permanent Magnetization”.]{ref}
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