Ch. XIV.] LOWER MIOCENE STRATA OF FRANCE. 217 



ual peculiarities of individuals ; but when once our specific determina- 

 tions are biassed by geological or geographical considerations, there is 

 an end of all reasonable hope of corning to consistent results. 



LOWER MIOCENE STRATA OP FRANCE. 



Remarks on classification, and where to draw the line of separation 

 behveen Miocene and Eocene strata. — The marine faluns of the valley 

 of the Loire have been already described as resting in some places on 

 a freshwater tertiary limestone, fragments of which have been broken 

 off and rolled on the shores and in the bed of the Miocene sea. Such 

 pebbles are frequent at Pontlevoy on the Cher, with hollows drilled 

 in them in which the perforating marine sheUs of the Falunian period 

 still remain. Such a mode of superposition implies an interval of 

 time between the origin of the freshwater limestone and its submerg- 

 ence beneath the waters of the Upper Miocene sea. The limestone 

 in question forms a part of the formation called the Calcaire de la 

 Beauce, which constitutes a large tableland between the basins of the 

 Loire and the Seine. It is associated with marls and other deposits, 

 such as may have been formed in marshes and shallow lakes in the 

 newest part of a great delta. Beds of flint, continuous or in nodules, 

 accumulated in these lakes, and aquatic plants called Charce, left their 

 stems and seed-vessels embedded both in the marl and flint, together 

 with freshwater and land shells. Some of the siliceous rocks of this 

 formation are used extensively for millstones. The flat summits or 

 platforms of the hills round Paris, and large areas in the forest of 

 Fontainebleau, as well as the Plateau de la Beauce, already alluded to, 

 are chiefly composed of these freshwater strata. Next to these in the 

 descending order are marine sands and sandstone, commonly called the 

 Gres de Fontainebleau, from which a considerable number of shells, 

 very distinct from those of the faluns, have been obtained at Etampes, 

 south of Paris, and at Montmartre and other hills in Paris itself, or in 

 its suburbs. At the bottom of these sands a green clay occurs, con- 

 taining a small oyster, Ostrea cyathula, Lam., which, although of 

 slight thickness, is spread over a wide area. This clay rests im- 

 mediately on the Paris gypsum, or that series of beds of gypsum and 

 gypseous marl from which Cuvier first obtained several species of 

 Paleotherium and other extinct mammalia,* At this point the ma- 

 jority of French geologists have always drawn the line between the 

 Middle and Lower Tertiary, or between the Miocene and Eocene 

 formations, regarding the Fontainebleau sands and the Ostrea cyathula 

 clay as the base of the Miocene, and the gypsum with its mammalia 

 as the top of the Eocene group. From that method of classification 

 I formerly dissented, agreeing with M. Deshayes that the fossils of 



* See below, Chapter XYI. 



