In contrast, surface soils which have remained surface soils for a long time often have high mineralisation, because the action of water, over a long period, causes iron compounds to migrate to the surface. For example, Australia has old soils, having had no glaciers recently or signifi cant mountains to be eroded. Some volcanic rocks or sands, known as black sands, may be highly mineralised and are found, for example, in a few USA mainland and Hawaii areas. These black sands or rocks are made of mostly magnetite, an iron oxide called ferrite. These typically produce almost entirely X signals, and almost no R. They are heavy, that is they have a high density, and can be identifi ed because they are strongly attracted to a magnet. Small roundish magnetite maghemite pebbles a few mm in diameter are also attracted to a magnet. These, for example, may be found in many Australian goldfi elds, but do produce signifi cant R signals.