LOWER MIOCENE OF CENTRAL FRANCE. 
231 
Paris, and at Montmartre and other hills in Paris itself, or in 
its suburbs. At the bottom of these sands a green clay oc¬ 
curs, containing a small oyster, Ostrea eyathula^ Lam., which, 
although of sliglit thickness, is spread over a wide area. 
This clay rests immediately on the Paris gypsum, or that se¬ 
ries of beds of gypsum and gypseous marl from which Cuvier 
first obtained several species of Paleotherium and other ex¬ 
tinct mammalia.* 
At this junction of the clay and the gypsum the majority 
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 cyathida clay as the base of the Miocene, and the 
gypsum, with its mammalia, as the top of the Eocene group. 
I formerly dissented from this division, but I now find that I 
must admit it to be the only one which will agree with the 
distribution of the Miocene mammalia, while even the mollus- 
ca of the Fontainebleau sands, which were formerly supposed 
to present a preponderance of affinities to an Eocene fauna, 
have since been shown to agree more closely with the fossils 
of certain deposits always regarded as Middle Tertiary at 
Mayence and in Belgium. In fact, we are now arriving at 
that stage of progress when the line, wherever it be drawn 
between Miocene and Eocene, will be an arbitrary one, or one 
of mere convenience, as I shall have an opportunity of show¬ 
ing when the Upper Eocene formations in the Isle of Wight 
are described in the sixteenth chapter. 
Lower Miocene of Central France. —Lacustrine strata, be¬ 
longing, for the most part, to the same Miocene system as the 
Calcaire de la Beauce, are again met with farther south in 
Auvergne, Cantal, and Velay. They appear to be the mon¬ 
uments of ancient lakes, which, like some of those no w exist¬ 
ing in Switzerland, once occupied the depressions in a mount¬ 
ainous region, and have been each fed by one or more rivers 
and torrents. The country where they occur is almost en¬ 
tirely composed of granite and different varieties of granitic 
schist, with here and there a few patches of Secondary strata, 
much dislocated, and which have suffered great denudation. 
There are also some vast piles of volcanic matter, the great¬ 
er part of which is newer than the fresh-water strata, and is 
sometimes seen to rest upon them, while a small part has ev¬ 
idently been of contemporaneous origin. Of these igneous 
rocks I shall treat more particularly in the sequel. 
The study of these regions possesses a peculiar interest 
very distinct in kind from that derivable from the investiga- 
* Bulletin, 1856, Journ.,^^ol. xii., p. 768. 
