Problems in determining paleointensities from very old rocks

Polyphase magnetizations are not uncommon in old rocks. To obtain reliable paleointensities, these magnetizations need to be recognized and separated so that the paleointensity determination can be derived from one of the phases of magnetization. Some of the techniques used to detect and separate (and sometimes isolate) the different phases are described by means of a few examples. Special attention is given to the chemical remanent magnetization which can be found in both sedimentary and igneous rocks. It appears that a sedimentary rock near an igneous contact is the preferferred specimen for reliable paleointensity determinations.