A priori and a posteriori geological mapping. The geological maps of the world by Ami Boué (1843) and Jules Marcou (1861)—the Australasian aspects

The paper describes and discusses the work done in producing the first geological maps of the world—by Ami Boué (1843) and Jules Marcou (1861)—and their later editions. Boué had a remarkably wide knowledge of geology through his own field investigations and his vast knowledge of the geological literature. The same may be said of Marcou. But their approaches to ‘global mapping’ were very different. Boué was greatly influenced by Élie de Beaumont and also the idea that geographical knowledge could in itself facilitate the formulation of geological hypotheses and make possible producing geological maps for areas that had not yet been examined by geologists. He did, however, also make use, where possible, of written reports of areas that he had not visited. He described his work as a priori mapping, with the use of analogical reasoning. Marcou's geological mapping likewise drew on his extremely extensive field experience and geological reading, but he did not colour in the parts of the globe for which he lacked any information. Coming eighteen years after Boué, there was inevitably more information available to Marcou. Their two efforts, procedures and results are examined for Australia and New Zealand, which neither of them ever visited. An attempt is made to identify the sources that each of them might have used. The paper provides reproductions of the maps that Boué and Marcou produced, and discusses the successes and failures of their enterprises.