From the analysis of the natural magnetism of rocks, a component--thermoremanent magnetism (T. R. M.)--can be isolated, which corresponds to the magnetism acquired by the rocks at the time of their formation. Results of observations based on the method show that the earth's magnetic field resembles the field created by a geocentric dipole, the axis of which, as related to the earth's crust, has changed considerably throughout geologic time. The intensity of the field has also varied although it is not certain to what degree. Observation of the nature and direction of the rock magnetism shows that the trajectory of the geomagnetic axis, in the course of time, corresponds to a complex curve with numerous reversals and points of intersection. Figures for the declination and inclination of the magnetic field obtained from Precambrian to Quaternary rocks of France and other areas of western Europe indicate that magnetic inclination during the Silurian and Devonian and the Permian and Triassic was slight; that magnetic declination was more often to the east than to the west; and that in the Permian, Triassic and Cretaceous variations in direction were of little importance and reversals were numerous, while in the Carboniferous and Neogene there was considerable variation in direction with fewer reversals.