The magnetic field anomaly map compiled by the C.N.R.S. in 1964, which is based on profiles spaced 10 km apart and at a height of 3 km, reveals a strong positive anomaly in the central region of the western Channel. The interpretation of this type of survey is considerably facilitated by the use of transformations which permit us to present two supplementary maps: (a) the first derivative of the magnetic field in the vertical sense and (b) that of the preceding function reduced to a pole, assuming initially that magnetized bodies are created by pure induction. From these two transformations in which the effect of the magnetic substratum is considerably enhanced, we have drawn contours which represent either the principal elements of the magnetic relief or the principal ranges of magnetization of a relatively flat magnetic substratum. In the subsequent phase of interpretation, namely a comparison with the geological information available, the presence of distinct bodies of uniform magnetization was assumed. Calculations of depth indices have also been made. From the results obtained, which are synthesized in this report, we have been able to formulate a hypothesis concerning the date of the emplacement of the structures responsible for the observed anomalies.