32 BELIQUUi AQUITANIC^E. 



successive limestones sloping away gently to the south-west (Section, p. 29, 

 k, i, 2, 3, 4), nowhere occurring all in one vertical section, and altogether probably 

 not having an aggregate thickness of much more than 200 metres. The lowest 

 of these four groups into which M. le Vicomte d' Archiac has divided the Cretaceous 

 strata of this Department seems to be wanting in the district traversed by the 

 Vezere, the series having there commenced with the beds characterized by 

 Exogyra columba the lower part of M. d'Archiac's Third Division. Of the 

 Third Group the uppermost beds are seen (under other limestones) to form the 

 banks of the Vezere at and near Montignac ; and they come out to the surface 

 of the country somewhat further up the stream. Limestones of the next highest 

 stage form the high ground near Montignac, and range beyond Perigueux on 

 the north, and to Sarlat and Gourdon on the south. 



The nearly flat uppermost limestones (Premier Etage of D' Archiac) form a 

 considerable extent of ground traversed by the Rivers Vezere and Dordogne, 

 on their way westward by Lime nil and La Linde towards Bergerac, halfway 

 between which last two places the Cretaceous strata disappear beneath Tertiary 

 beds (I) of limestone (of freshwater origin), sands, clays, and ironstone. 



The Cretaceous limestones of the Department of the Dordogne are remarkable 

 for the large rough Bivalve shells belonging to the family Rudistes, particular 

 forms of which abound at four or five different levels in the series. The lime- 

 stones are often oolitic (or rather pseud-oolitic) in structure; that is, they 

 consist of roundish fragments of Shells and Polyzoans cemented with carbonate 

 of lime (occasionally crystallized). Sometimes the particles are but slightly 

 cemented, and the rock is friable. Occasionally the fragments of Polyzoa have 

 not been much triturated ; and in some cases they are merely broken stems lying 

 matted together. Similar varieties of Polyzoan limestone occur in the Chalk 

 of Touraine, Maestricht, and Denmark. 



The Elint in the Chalk of the Dep. de la Dordogne is mostly only so much of the 

 Cretaceous stratum silicified, the particles of Polyzoa, the Orbitoides, and other 

 organic remains being still in place and retaining their characteristic structures. 

 Even Fish-teeth (Otodus), in a block of Flint from near Le Moustier, have been 

 altered into Flint, excepting a thin external pellicle. There is also Flint showing 

 a further progress of mineralization, in which the constituent organic remains of 

 the limestone have been more and more removed from sight by the increased 

 homogeneity of the pseudomorphous silex, as is usually the case with the flint of 

 Northern France and of England. 



The Dordogne limestones are thrown into undulations, and traversed by cracks 



