The airborne magnetometer developed during World War II for tracking submerged submarines, combined with radio-location or other means for accurately locating the airplane, constitutes a new tool of modern geophysics by means of which magnetic surveys can be made with amazing speed over land or water. This memoir presents a method of interpreting magnetic surveys which, if extensively applied, might yield new knowledge regarding the structure of the earth’s crust and shed light on specific problems of regional geology, such as the maximum depth of sedimentation in geologic basins. It can be applied also to the delineation of buried contacts and the location of probable areas of rock differentiation and of mineralization in regions where the igneous rocks crop out. 1 Airborne magnetic surveys are not essentially different from land magnetic surveys of the vertical magnetic intensity carried out in the past with the Schmidt-type magnetic balance. There is no reason to expect that airborne magnetic surveys can yield more geological information, or that what information they might give is different in kind. There is some benefit derived from removing the magnetometer a few hundred feet above glacial till and other near-surface magnetic disturbances which occasionally affect the readings of the land magnetometer. Except when prospecting in regions where the igneous rocks crop out, this factor is of minor importance. Surveying from more than one altitude is of little value because, if the magnetic intensity has been surveyed at a given height above the surface of the magnetic rocks, it is