The frequent co-existence of strong magnetic and gravity anomalies directs attention to the properties of igneous rocks which may account for them. Density data are direct and unambiguous but magnetic data are much less certain. On the assumption that it is determined by disseminated magnetite, the susceptibility can be estimated from the proportion of magnetite in the rock. The possibility of estimating magnetite content from chemical analyses is examined and while not very satisfactory, it probably gives relative values of some merit. From published chemical analyses the magnetite content and probable intensity of magnetization are calculated for some 1450 samples of igneous rocks, for which density values are also given. The values and resulting contrasts are analyzed statistically and tabulated for the C.I.P.W. and the Iddings methods of classification. The contrasts, thus determined, are in reasonable agreement with expectations from the analysis of magnetometer and gravimeter surveys.