THE exposed portions of the Canary Islands largely comprise Cainozoic volcanic rocks erupted sub-aerially in successions essentially specific to each island, though with somewhat complex interrelationships1. On three islands, Fuerteventura, La Palma, and Gomera these rocks rest unconformably on older formations which are similar enough to suggest that a common basement may exist. The largest area of the basement complex rocks is the Betancuria Massif2–4, Fuerteventura. Gastesi4 has suggested that similarities exist between the Betancuria Massif and other ophiolite complexes (such as the Troodos Massif of Cyprus5,6) generally held to be representative of oceanic crust formed at certain constructive plate margins.