Both Boué and Marcou were ‘polymathic’ geologists with extraordinary knowledge of the geological literature. In 1843, Boué exhibited a hand-coloured geological map of the world at a scientific meeting in Graz. Subsequently it was formally published in several French and English versions. The map(s) coloured virtually all land areas of the globe. How did Boué do it? He relied on reports from as many parts of the world as possible, utilised the tectonic theory of Élie de Beaumont but also made far-fetched analogies. And some of his information was essentially based on guesswork. Nevertheless, his achievement was remarkable and it enables us to see what was known of world geology in the mid-nineteenth century. And one can observe the development of his work through the map’s several editions. The paper primarily focuses on Australasia and identifies the sources that Boué used for that part of the world and the extrapolations that he made. The two versions of Marcou’s maps reveal the development of geological knowledge over the two following decades. His reading was no less remarkable, but he left some areas of the world blank and did not try to ‘guess’, where no information was available. The ‘rationale’ for producing such maps is briefly discussed.