OF THE PARIS BASIN. 



of the Tertiary series — a position to which the late Prof. Hebert 

 assigned them. We are incHned to agree with Prof. Prestwich,* M. 

 Watelet,t and others, that the Rilly Beds should be placed at 

 the base of the Lignites and Mottled Clays. This raises a question 

 as to the age of the Marls of Meudon (p. 5). 



Near Sezanne, what is generally regarded as the Rilly Limestone, 

 contains an abundant flora described by M. de Saporta.t Above 

 the Chalk comes a sandy unstratified bed, on which rests a 

 travertine about sixteen feet in thickness, and here it is that the 

 fossil plants were found. From the general appearance of the 

 deposit and its included remains, French geologists regard the 

 Sezanne limestone as the site of an ancient cascade which was 

 surrounded by large trees. Some of the characteristic plants 

 found are alluded to in the paleeontological section (see p. 62). 



6. PLASTIC CLAY. 



This formation, which does not differ in its essential characteristics 

 from our Reading plastic clay, is of very variable thickness in the 

 Paris Basin, being only a few feet in some parts, but as much as 

 162 feet below St. Denis. M. Ch. d'Orbigny has shown § that it 

 commences with the Conglomerate of Meudon.. formed of three 

 beds : — 



1. Grev clay with plant remains. 



2. Laminated clay with gypsum, lignite and fresh-water shells — 



Unto antiquus, Physa Heberti, Viviparus suessioniensis. 



3. Rolled fragments of chalk, and pisolitic limestone, with Viviparus, 



Diplocvnodon depressifrons, Gastornis parisiensis, Coryphodon 

 anthracoidens. 



Above this, omitting unimportant local deposits, comes the 

 true Plastic Clay, which'is divided into two main parts, the glaises 

 and \\i& fausses glaises ; these beds are separated by a stratum 

 of fine clayey and lignitiferous quartzose sand, about seven feet 

 in thickness, with pyritous concretions. M. de Lapparent says || 

 that this sandy bed becomes of more and more importance as it 

 goes towards the Soissonnais, where the Plastic Clay in its turn 

 is not well developed. These sands are about thirty-two feet in 

 thickness in the forest of Compiegne. In ascending order we then 

 have a striped clay with Ostrea bellovacensis, and sometimes a fresh- 

 water limestone. The Plastic Clay is largely worked for tiles, and 

 other building purposes. 



* Prestwich, Bull. Soc. G^ol. Fr., ze. s($r., t. x. (1853), p. 300; also Quart. Journ. Geol. 

 Soc, vol. xliv. (t888), p. gg. 



t Watelet, liTeiit. Soc. Malac. Belg., t. x. (1875), p. 113. 



J Saporta, Mem. Soc. Geol. Fr., 2e. s6r., t. viii. (1851). 



§ Ch. d'Orbigny, Bull. Soc. Geol. Fr., i e. s(5r., t. vii. (1835), p. 281. 



II De Lapparent, Tr. de Geol., 2 ed. (1885), p. 1130. 



