OF THE PARIS BASIN. 3 1 



The formation is naturally divided into two groups : (i) the 

 lower marine and (2) the upper lacustrine. Taking advantage of 

 this circumstance M. Uollfus conveniently abridges the sub- 

 divisions as follows : — 



Divisions of the Gypsum beds and marls. 



T ■ { d. Lacustrine and marshy beds : blue and white '' subra- 



I.acustrine ) ^ " i 



. , < gvfiseuses marls. 



pel 10 . 1^, ;y|;jrsh-bed horizon : First mass of gypsum. 



,T • ( l)- Brackish and marine beds: 2nd, 3rd, and 4th masses of 



Marme \ , > -. i -r 



P^'^"^ ■ ( a. Marine beds : green sands " infra-gypseux " of Monceaux. 



(a.) Green sands of Monceaux. — These sands were so 

 called by M. Ch. d'Orbigny from the circumstance that they were 

 typically developed in the railway cutting at Monceaux on the 

 Saint Germain line. They contain Potamides tricarinatiis, P. 

 concavus, P. Cordieri, Lucina saxorum, Cardium granulosum, and 

 other fossils clearly indicating their close affinity with the Sables 

 Moyens, in spite of the intercalation of the St. Ouen limestone. 



{b.) Marine Gypsum beds. — The lower part of the gypsum, 

 which, as we have seen, contains certain marine horizons, is com- 

 posed of alternating beds (thin, but generally of considerable 

 extension) of marl and crystalline gypsum. The fourth, or oldest, 

 bed of gypsum can be easily studied in sections near Argenteuil 

 and Romainville ; it is confmed to the centre of the basin. The 

 third bed terminates (in ascending order) with the Z«r/«rt-marls 

 which contain an interesting fauna, Lucina inornata, Corbula 

 subpisum, Corbuloinya JVysti, Nucula capiliacea, Planorbis spiru- 

 loides and Potamides Roissyi* The majority of the fossils are well- 

 known Eocene types ; but the presence of Corbula subpisum and 

 Corbulomya Nysti foreshadow the incoming of the Oligocene. 

 The second bed of gypsum terminates the marine facies of the 

 formation. Fossils are very rare in the included beds ; but we 

 may cite Potamides pleurotomoides^ P. tricarinatus, and Mcsalia 

 incerta, all three of which are characteristic Eocene species found 

 by M. Goubertf in the yellow marls. 



The marine gypsum and marls have been cited, amongst other 

 places, as occurring at Montmartre, Bry-sur-Marne, Argenteuil 

 (Butte d'Orgemont, Carriere Bast. Fig. 11), Montigny, Herblay 

 and Frepillon. 



MM. Raulin+ and Eck§ have shown that the limestone of 

 Ludes, near Rheims, is approximately of the same age as the 

 " infra-gypseuses " P/toladomya-m3ir\s of Paris. 



(c.) Lacustrine Gypsum beds. — The first mass or lacustrine 

 gypsum beds are the most developed (sixty-five feet at Montmartre), 



* For an excellent description of the marine marls of the Gvpsum formation see Carez, 

 £ul/. S'C. Geol. Fi-., ae. s6r., t. vi. (1S78), p. 187. 



t Goubert, Bull. Soc. Geol. Fr., 2e. s6r., t. xvii. (i860), p. 600 ; and t. xxili. (iS66),p. 340. 

 X Raulin, Bull. Soc. Geol. Fr.. le. siir., t. xiv. (1842), p. 42. 

 § Eck, Pr. Verb. Soc. Malac. Bel^., t. vii. (1S7S), p. v. 



