Mapping of the Paleozoic of the Central Pyrenees by students in geology of Leiden University under the directorship of Prof. Dr L.U. de Sitter started in 1948 with the Arize massif. Since then the survey has shifted first eastwards, 1949 St. Barthélemy massif; then westwards, 1950 Salat valley; 1951 Riberot valley (sheet 2); 1952, Garonne valley (sheet 1), and then southwards, 1953 and 1954 Valle de Arán, then south again to the Pallaresa and the Sègre (1955—1957). The survey has been restricted almost exclusively to the Paleozoic because the Mesozoic had been mapped by Casteras (1933) in the north and by Misch (1934), in the south, therefore only the Paleozoic rocks in their nonmetamorphic and metamorphic state have been differentiated on the map, whereas the Mesozoic rock contours except part of the Triassic, have been taken over in a simplified form from the 1 : 80.000 sheet Foix of the Carte géologique de France. Many internal reports and maps have accumulated in the files of the Geological Institute in Leiden of which only a few have been published. Often preliminary surveys have been succeeded by further detailed fieldwork by graduate students, other regions have been worked over again by ourselves so that now a start can be made with a final comprehensive series of maps on a scale 1:50.000 of which Sheet 3, Ariège, France, containing mainly the satellite massifs of Arize, Trois Seigneurs and St. Barthélemy (see fig. 1) is the first to be published.