Part I deals with the magnetic instruments, methods and anomalies found in the areas of São Pedro-Xarqueada. Askania vertical and horizontal intensity magnetometers, and stationary instruments for photographic registration of diurnal and other changes, were used. Most of the anomalies were found to be related to diabase sills, lacoliths and dikes. Depth calculations by the methods of Tiberg, Haanel and Thalen did not correspond closely with depths found in wells, but were of the same order of magnitude. Depth to basement rocks could not be calculated from the magnetic results, and the authors suggest the use of seismic methods. Tables showing the magnetic properties of the rocks in the area are included.