Ch. XIV.] LINE BETWEEN MIOCENE AND EOCENE. 219 



very distinct geological position above the typical Eocene series of the 

 Paris basin, and of which the equivalents- at Mayence had long been 

 recognized as Miocene. M. Hebert also published, in 1855, a map 

 descriptive of the areas of two tertiary seas, which succeeded each 

 other in the Paris basin, — the first that of the Calcaire Grossier, and 

 the second that of the Fontainebleau Sands, — showing how marked is 

 the want of coincidence between them ; a fact which implies the 

 occurrence of great geographical changes in the interval of time be- 

 tween the two eras compared. In the explanation of his map he 

 gives his reasons for regarding the zone of Cerithium plicatum, or 

 that of the Fontainebleau Sands, as the most convenient line of de- 

 marcation between Lower and Middle Tertiary, or between Eocene and 

 Miocene.* 



When I was hesitating as to the course which it would be most ex- 

 pedient to take in drawing the line between Eocene and Miocene, M. 

 Lartet, the distinguished French zoologist, whose writings on fossil 

 mammalia are of such acknowledged value, remarked to me that although 

 the fossil testacea of the Fontainebleau Sands show a preponderance 

 of affinities toward an Eocene fauna, and small connection with the 

 faluns of Touraine, yet, on the other hand, the freshwater " Calcaire 

 de la Beauce," immediately overlying the Fontainebleau Sands, and 

 other lacustrine formations in Auvergne and Central France, as well as 

 the fossiliferous strata of the Mayence basin, cannot be included in the 

 same Eocene system without doing violence to paleontological princi- 

 ples. The grouping of the fossil mammalia, he observed, becomes less 

 natural by such an arrangement ; for not only many genera, but even 

 some species, are found on both sides of the arbitrary fine of demarca- 

 tion thus drawn between Eocene and Miocene. The genera Dorca- 

 tkerium, Cainotherium, Anchitkerimn, and Titanomys, for example, 

 and Rhinoceros incisivus and others, would thereby be made common 

 to Eocene and Miocene. 



Other arguments drawn from fossil botany in favor of uniting the 

 Gres de Fontainebleau and faluns in one group will be more fully set 

 forth in the next chapter, when I treat of the tertiary strata called 

 " Molasse " in Switzerland, and of the German Brown Coal. 



My unwillingness to include the Fontainebleau sands and other 

 strata of the same age in the Miocene Epoch arose partly from the 

 necessity thereby incurred of abandoning for such deposits the defini- 

 tion which I had already given of the term Miocene as implying that 

 a marked proportion, though a minority, of the fossil shells belong- 

 to living species. I had felt myself obliged, even in 1833, to disre- 

 gard this difficulty, when, in the first edition of the " Principles of 

 Geology," I classed the strata of the Mayence basin as Miocene, con- 

 ceiving that, although almost every species of shell was extinct, they 

 had more affinity with the Falunian than with the Eocene formations. 



* Bulletin, 1856, torn. xii. p. 760. 



