345 I'he Ougocexe of the Southern United States 35 



When t3'pical the limestone is white and conipa(5l, with 

 silicified laj^ers which are sometimes used for millstones. 



The fauna is poor. The only really characfleristic fossil is 

 Nystia Duchasteli, but a few Limnaeas have been found. From 

 the presence of Nystia Duchasteli, the limestone of Castillon is 

 thought to be the equivalent of the Brie limestone of the P£.ris 

 basin. 



The Ostrea clays. The clays charadlerized by Ostrea longi- 

 rostris and O. cyathtda (O. girondica) usually precede the lime- 

 stone with Asterias. The oyster beds, themselves, rest either on 

 the limestone of Castillon or on the sandstone of Fronsac. The 

 basal layers of the oyster beds are rich in foraminiferas. 



Asterias or Boiirg limestone. The limestone characfterized 

 b}^ Asterias is typical at Bourg. Since this is a coarse-grained, 

 yellowish limestone, Deshayes, Brongniart and others regarded it 

 as equivalent to the Eocene limestone of the Paris basin (calcaire 

 grossier). Des Moulins named the Asterias limestone from the 

 number of Crenaster Icevis that it contains. But although he 

 observed the close resemblance of the Bourg fauna to that of the 

 o\'ster marls and green clays of the Paris basin, he still considered 

 the limestone as an Eocene formation. 



M. Raulin,'^^ in 1843, in an article on the tertiaries of the 

 Gironde, and in all his later publications, attempted to discrimi- 

 nate between the Bourg limestone and that of St. Micaire, consider- 

 ing the former as Eocene and the latter as Miocene. But Delbos,t 

 Tournouer,J Matherson|| and Linder§ proved that these lime- 

 stones are identical and equivalent to the sands of Fontainebleau 

 of the Paris basin. 



*Nouvel Essai d'une classification des terrains tertiaires de I'Aquitaine, 

 1848; Resultat des Excursions faites dans le partie O. du D^pt. des Landes, 

 1864-9. 



t Rapport sur une excursion geologiques aux environs de Blaye, Act. 

 Soc. Linn. Bord., t. XIII, 1870. 



X Recensement des Echinodermes du Calcaire a. Asteries, Extrait des 

 AHes dc la Soc. Linn, de Bord., t. XXVII, 1870. 



II Notes sur les depots tertiaires du Medoc et des Environs de Blaye, etc., 

 BiUl. de la Soc. Geol. dc Fr., 2enie ser., t. XXIV, p. 197, 4 Fev., 1867. 



'i Observations sur les depots tertiaires du Medoc et du Blayais dans le 

 Dept. de la Gironde, 1873, Coniptes rendues de la Soc. Linn, de Bord., 1874. 



