Three main stratigraphic scales are used regionally for the Carboniferous System: U.S.A., Western Europe, and U.S.S.R. Correlation problems among these three classifications are far from having precise solutions. It is necessary for each absolute age measurement, to point out the regional classifications to which the analyzed sample belongs, because in any case the measured age cannot be considered as having a world value. The partial results acquired by now are as follows:

(1) Devonian-Carboniferous boundary, 330 m.y. (Western Europe, Armorican Massif); (2) Mississippian-Pennsylvanian boundary, less than 320 m.y. (U.S.A., under the gap between the two subsystems); (3) Bashkirian-Moscovian boundary, 308 m.y. (U.S.S.R., Donetz basin); (4) Middle-Upper Carboniferous, (Westphalian-Stephanian) boundary, 300 m.y. (Western Europe, Cevennes basin); and (5) Carboniferous-Permian boundary, 290 m.y. (U.S.S.R., Akchatan Granite).