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 



