The Colours of the Earth. Logic of Colour-coding Systems in 19th Century Geological Maps

International audience"I have oftentimes wished for a mappe of England coloured according to the colours of the earth". The idea of drawing maps showing information about rocks by colours, expressed here in 1685 by the English antiquary John Aubrey, was not achieved until the second part of the XVIIIth century. Such use of colour was introduced in the cartography of Germanic countries, which possessed an advanced knowledge in mining (geognosie). In the XIXth century, colour became an essential variable to visually express subsoil phenomena, wether applied by hand or by mechanical means. The colour codification followed the evolution of knowledge about the nature of the subsoil, in this period of emergence of the geological science.We will try here to analyze the transition to this so-called chorochromatic cartography and to clarify the logic of the conventions instituted by cartographers. This study will take us until the end of the XIXth century, when geologists, within the framework of their international congresses, tried to standardize the colour code