The type areas for Oligocene stratigraphy lie mainly in France— among the lagoons left by the retreat of the Eocene seas. The following is the classification of the Oligocene now generally adopted (reading downwards) :— The Sannoisian Stage (Sannois hill, near Argenteuil, north-west of Paris) is represented by the famous series of supra-gypseous marls. At the base are blue marls, laid down in salt lagoons, then white marls with fresh-water horizons represented by beds with Limnaea strigosa, then Cyrena marls and finally a group of green marine marls with Cytherea incrassata. The latter is an horizon distinguishable over wide areas. South-east of the Marne, the marls are replaced by the fresh-water limestones and "meuliere" of Brie. "Meuliere" is a porous, siliceous rock, formerly much worked for building stone.
The Stampian Stage (Etampes, Paris basin) is the most im portant and most easily recognized of the Oligocene horizons in the Paris basin. At the base are the widespread oyster marls (Marnes a Huitres) containing especially Ostrea cyathula and 0. longirostris, followed by the well-known Fontainebleau sands with an average thickness of 125 feet. These are the "sables superieurs" of the older French geologists. They are generally white and very pure, and are exploited as glass-sands. Except at a few points in the neighbourhood of Etampes (Jeurre, Morigny, Pierrefitte, etc.) they are rarely fossiliferous but there contain Pectunculus obovata, Cytherea splendida. Consolidated blocks (gres de Fontainebleau) comparable with the "sarsens" of the English Eocene are common. After the deposition of the marine Fontainebleau sands, the sea left the Paris basin ; a huge lake covered the south-west of the Paris basin. In this were deposited the fresh-water Beauce limestones, representing both the Chattian and Aquitanian. The lower part (Chattian) of this limestone is now usually called the Etampes limestone of which the Meuliere de Montmorency is a lateral equivalent.
The Aquitanian Stage (Aquitaine, France) owes its name to its development in the great Aquitaine basin of south-western France where there are marine limestones and brackish water intercala tions in a series of shelly sands with Melongena, the whole passing southwards into a mass of conglomerates (poudingues de Palas son).
In Britain, if one excepts the possible Oligocene age of the beds of the Bovey Tracey basin in Devonshire and of some of the sediments intercalated with the lavas of the west of Scotland (leaf beds of Mull), Oligocene beds are found only in the Hamp shire basin. The classic exposures are in the cliffs of the Isle of Wight. The base is ill-defined and may be taken perhaps at the base of the Middle Headon beds (brackish and marine). These are succeeded by the fresh-water clays, marls and limestones of the upper Headon beds, then the Osborne and Bembridge beds (a series of brackish and fresh-water sands, clays, marls, and limestones) and finally the Hamstead beds, estuarine and fresh water below and marine above. The correlation of the Hampshire
sequence with the French stages is still uncertain ; probably the highest marine Hamstead beds are equivalent to the Oyster marls of the lower Stampian. In this case the whole of the Hampshire Oligocene, except the highest beds, is Sannoisian.
In Belgium the Sannoisian sands and clays are important and extensive and known as the Tongrian. The Stampian is represented by a thick series of clays—the Argiles de Boom (Rupelian) whilst the Chattian seems to be absent unless represented by pockets of marine sands in the Ardennes massif. There is no deposit corresponding to the Aquitanian. In northern Germany and Den mark the Oligocene is, in contrast to the Eocene, thick, impor tant, and fossiliferous. The Sannoisian (or Lattorfian) is repre sented by marine sands with Nummulites germanicus; the Stamp ian continues the Sannoisian transgression and occurs in the Mainz basin and even as far south as Alsace. The Chattian (or Casselian) derives its name from the Chattes, an ancient tribe of the Cassel country and consists of fine glauconitic sands well developed in the Cassel gulf. In the Mainz basin are Cyrena marls and Cerith ium limestones. The Aquitanian is, probably, represented by the "Littorinellenkalk" of the Mainz basin, difficult to separate from the overlying Miocene. Brown coal is important in the Oligocene of Germany. Continental Oligocene beds are found in several basins of the central European continent, notable in the Rhone basin (deposits of Alais, Aix, Marseille, etc.) and in the trenches of the Loire and Allier.
In the alpine geosyncline it is probable that the only Oligocene are Sannoisian—notably the sandstones with small nummulites above the Priabonian and the higher beds of the Flysch. Oligocene deposits occur in the Carpathian region and the Tirol, in the Spanish Pyrenees and the flanks of the Cantabrian mountains, in Calabria, Dalmatia, Istria, and Bosnia. Glauconitic sands occur in south Russia, Flysch in the Caucasus, marine deposits in the Aral-Caspian region, Armenia, and Persia. Oligocene rocks are known also in north Africa, Algeria, Tunis and Egypt In general Oligocene beds succeed Eocene in the epicontinental and geo synclinal gulfs around the continents of Asia (including the Nan series of India and the lower Peguan of Burma) and the Americas Of special interest are the continental Oligocene deposits of North America, including the White River beds of South Dakota. (See references given under EOCENE.) (L. D. S.)