OF THE PARIS BASIN. 29 



tuberculations are beautifully preserved on the P. pleurotomoides. 

 Melongena minax is found, though somewhat rarely, but Melongena 

 siibcarhiata and Tritonidea polygona are met with just above the 

 Potamides-htdi in large numbers. 



Below the Fotamides-htd, Avicula Defraficei is found, and this 

 is an important fossil from a stratigraphical point of view, being 

 characteristic of this horizon. 



Much general information on the Sables Moyens is given in 

 the paper by M. G. DoUfus already cited and in the Bull. Soc. 

 Geol. Fr., 3" sen, t. vi. (1878), pp. 243, et sqq. The junction with 

 the Calcaire Grossier is well shown in the Mery section and the 

 neighbourhood of Le Guepelle. 



At the last-mentioned locality, also, the three horizons of the 

 Sables Moyens are well developed. 



General Section at Le Guepelle (near Survilliers* ). 



{. Clay bed. 

 Upper < Tabular sandstone with Potamides tricar matin;, P. pleurotomoides, 



{ P. Cordieri and Avicula De/rancei=\lor\zox\ of Mortefontaine. 



( Sands without fossils. 

 Middle < Sands with Cerithium tuberculosum, C. mutadi/e, &c.=Horizon of 



( Beauchamp. 



j Calcareous sands, very fossiliferous, with Ampullina parisiensis, 

 y J CalliompJialus moniliferum, Sycum hulbiforme, Volutilithes labrella, 



J Potamides Bouei, Bavania lactea, Cardita sulcata, Corbula gallica, and 



( Caihafiassa=Wor\zon of Auvers. 

 The Upper Calcaire Grossier is seen in the railway cutting near by. 



12. LIMESTONE OF ST. OUEN. 



The passage from the Sables Moyens to the St. Ouen limestone 

 is a very gradual one, and the lacustrine character of the latter 

 bed renders it difficult to institute a correlative comparison 

 between it and the marine Sables Moyens. We have seen that the 

 lacustrine facies set in during the preceding period, as witness the 

 fresh-water limestones of Ducy, of Nanteuil, in the railway cutting 

 at Mery, &c., and that its fossils are partly characteristic of the 

 Beauchamp sands and partly of the St. Ouen limestone. 



The St. Ouen limestone {'^travertin i?iferieur" of some 

 authors) is in the aggregate from thirty two feet to sixty-five feet in 

 thickness, and is white and marly, with thin sandy beds and fresh- 

 water fossils. Its lowermost bed lies on the zone of Avicula 

 Defranceioi the Sables Moyens. M. G. Dollfusf has divided this 

 limestone into two parts, following the separation by a marine 

 band having the closest affinities with the Avicula-zono. ©f 

 Mortefontaine. The two beds of limestone are very similar to 

 each other, but the lower is compact, while the upper is marly 

 with numerous siliceous nodules, characterised by the presence of 

 Linuma. Well-known fossils, such VLsLinifuea longiscata, Dissostonia 



* From Dr. Hovelacque's notes. 



t Dollfus, Bull. Soc. Geol. Fr., 36. s6r., t. vi. (1878), p. 42. 



