Marine magnetic surveys reveal two important positive magnetic anomalies, coincident n location and trend with the continental margin. The form and spatial relationship of these and other positive anomalies suggest thay they represent two buried marginal ridges of metamorphic basement rocks with an intervening E.-W. fault at 56°N. The southern ridge is believed to terminate at 54° 50’N along an ENE.-WSW. line which could represent the offshore course of the Great Glen Fault. To the S., a subdued and generally negative anomaly pattern possibly implies a fault-bounded depositional trough occupying Donegal Bay.