Earth resistivity and geological structure

The relation which the resistivity of the earth's crust bears to the geological structure has been studied with the aid of many experimentally obtained data. Although the resistivity is found to vary between wide limits, and not always to be the same even for the same types of rock formation in different locations, it has been found that the variation for any particular material is within more narrow limits, and that the older materials generally have the higher resistivities. The results of the studies are of value in several types of electrical calculations.