208 
MIOCENE PERIOD. 
[Ch. XV. 
compact fresh-water limestone, of slight thickness, which is per- 
forated on the upper surface by marine shells, for the most 
part of extinct species. It is evident that the space must have 
been alternately occupied by salt and fresh water. First, a 
lagoon may have been formed, in which the water may have 
become fresh ; then a barrier of sand, by which the sea was 
excluded for a time, may have been breached, whereby the 
salt water again obtained access. 
Eocene strata in the Bordeaux basin. — The relations of some 
of the members of the tertiary series, in the basin of the Gironde, 
have of late afforded matter of controversy. A limestone, re- 
sembling the calcaire grossier of Paris, and from 100 to 200 
feet in thickness, occurs at Pauliac and Blaye, and extends on 
the right bank of the Gironde,, between Blaye and La Roche. 
It contains many species of fossils identical with those of the 
Paris basin. This fact was pointed out to me by M. Deshayes 
before I visited Blaye in 1830 ; but although I recognized the 
mineral characters of the rock to be very different from those 
of the Miocene formations in the immediate neighbourhood 
of Bordeaux, I had not time to verify its relative position. I 
inferred, however, the inferiority of the Blaye limestone to the 
Miocene strata, from the order in which each series presented 
itself as I receded from the chalk and passed to the central 
parts of the Bordeaux basin. 
Upon leaving the white chalk with flints, in travelling from 
Charente by Blaye to Bordeaux, I first found myself upon 
overlying red clay and sand (as at Mirambeau) ; I then came 
upon the tertiary limestone above alluded to, at Blaye; and 
lastly, on departing still farther from the chalk, reached the 
strata which at Bordeaux and Dax contain exclusively the 
Miocene shells. 
The occurrence both of Eocene and Miocene fossils in the 
same basin of the Gironde, had been cited by M. Boue as a 
fact which detracted from the value of zoological characters as 
a means of determining the chronological relations of tertiary 
