In 1987 he published Before Columbus: Exploration and Colonization from the Mediterranean to the Atlantic 1229–1492, a study of the earliest phase of European imperialism when Europeans left the Mediterranean and colonized the islands along the northwest coast of Africa.
E en 1987, Before Columbus: Exploration and Colonization from the Mediterranean to the Atlantic 1229-1492 ("Antes de Colón: exploración e colonización do Mediterráneo até o Atlántico 1229-1492"), un estudo da primeira fase do imperialismo europeo cando os europeos deixaron o Mediterráneo e colonizaron as illas ao longo da costa noroeste de África.