26 G. F. Harris — Geology of the Gironde. 



Gironde, but also in many river-valleys to the south and west of 

 Bordeaux. The upper and lower Aquitanien have generally a fresh- 

 water facies, whilst the middle is marine ; but this does not hold 

 good in the western portions of the area under consideration. As 

 they are traced westwards, the fresh-water beds gradually become 

 more marine in character, and, as might naturally be supposed, it is 

 next to impossible to define the three assises where this is the case ; 

 the latter have, therefore, only a local value in the Agenais. 



The Aquitanien beds presenting the greatest interest, perhaps, 

 to English palaeontologists are those found in the vallej's round 

 Saucats and La Brede, about twelve miles south of Bordeaux, for the 

 majority of the Bordelais Oligocene fossils come from thence. The 

 district has been most admirably described by M. Tournouer, 1 and 

 the following useful zones have been established, the oldest, as usual, 

 being placed at the bottom : — 



5. Marly lacustrine limestone, with Helix girondiea, Limnea girondiea and Dreissina 

 Brardi. 



4. An argillaceous bed with Potamides in one place and a marine bed in another. 

 3. Lacustrine limestone of Saucats. 



2. A loosely compacted sandstone corresponding to the Bazas sandstone, and 

 known to Bordeaux geologists as " roche sableuse jaune." 



1. Blue and white clays with Neritina Ferusaci (Nerita picta). 



These form the first (lowest) five zones of what are known as " les 

 couches faluniennes " of that author. 



The bed No. 4 at Lariey (about two miles north-east of Saucats) 

 is very fossiliferous, containing many species originally called 

 " Lower Miocene." Under M. Fallot's guidance, I succeeded in 

 obtaining a large number from a few small openings, amongst which 

 may be mentioned Cyllenina baccaia, var. minor, Proto Basteroti, 

 Calyptrcea sinensis, Charaa Brocclii, Lucina dentata, Terebra (Hastula) 

 cinerea, and Cytherea undata. 



The same horizon is met with under another guise in a road- 

 section on the way to Son, between Lariey and Saucats, but nearer 

 the former place. The locality is sometimes referred to as "Moulin 

 de l'Eglise." In this road-cutting one sees zones 3, 4, and 5. Here 

 zone 4 has changed into a brackish-water fauna chiefly composed of 

 Potamides. 



Miocene. 



This comprises the Middle and Upper Miocene of old authors. 

 Beds of this age occupy the greater portion of the western half of the 

 Gironde; but they are so completely covered by superficial deposits, 

 mostly by the " Sable des Landes," that they are visible at but very 

 few points, situated almost exclusively in river-valleys which have 

 been cut by ordinary river action, down through the " sable " alluded 

 to. The beds are divided into four parts, forming, in upward succes- 

 sion to the Bordelais Aquitanien beds, a further instalment of "les 

 couches faluniennes," and numbered as follows : — 



9. Faluns of Salles and Sime, with the mollasse of Martignas. 



8. Faluns of Saucats and Cestas. 



7. Typical falun of Leognan, and those of Cassagne and Lagus. 



6. Mollasse of Leognan, with the lower faluns of Leognan and those of Giraudeau 

 and Peloua. 



1 " Bull. Soc. Geol. de France," 2 e ser. tome xix. p. 1039. 



