PRESTWICH BRITISH AND FOREIGN TERTIARIES. 



219 



Tertiaries, and underlies the Lignites of the Soissonnais — has been 

 considered definite. (See PI. VIII., Diagram, str. I, m, n, o.) 



But there seems to be some evidence in France, as in England, that 

 the lignites alternate occasionally with the mottled clays, and that 

 fluviatile clays with the Cyrenay &c. are sometimes found under 

 beds of these clays, although the occurrence of such phaenomena 

 abroad is much less frequent and distinct than it is here. Neverthe- 

 less some such instances may be detected in the works of M. Buteux 

 and of M. Graves, notwithstanding that the impression of the authors 

 seems to be that the true " argile plastique " always underlies the 

 lignite and fluviatile clays, and the evidence to the contrary is cer- 

 tainly scant. The former author, however, gives one distinct section 

 at Marche-le-Pot, where four beds of variously coloured (mostly red, 

 and mth pebbles in one bed) plastic clay, together 9 to 1 5 feet thick, 

 overlie 3 feet of Lignites with bands of other clays ; the sections also 

 at Mont Soufflard, Lihons, and Hallu show an apparent intermixing 

 of lignites, mottled clays, and pebble beds, in one place (Hallu) over- 

 lying sands with Cyrena and Cerithium^ . 



M. Graves gives a section at Bonvillers, where 7 feet of well- 

 marked mottled red and purple clay overlie clays with Cyrena, 

 Ostrea, &c. and lignites. At Canly also, ^\ feet oi plastic clay re- 

 poses on freshwater clays and lignites ; and at Coivrel the shells seem 

 sometimes to be imbedded in mottled clays f. (See also p. 245-7.) 



With reference now to the organic remains of the fluviatile clays 

 and lignites considered apart, the evidence of the synchronism of the 

 French and English series has, as before mentioned, been often ob- 

 served upon and appears well founded. In the London district, the 

 group of shelly clays, sands, and pebble beds, with lignites, of Black- 

 heath, Woolwich, New Cross, and Lewisham contain forty-two spe- 

 cies of mollusks, the following eighteen of which are also found in 

 the lignites of the Oise and the Soissonnais : — 



Area depressa, Soiv. 

 *Cyrena cuneiformis, Fer. 



tellinella, Desk. 



Nucula fragilis, Desk. 

 *Ostrea Bellovacina, Desk. 



tenera, Sow. 



Pectunculus terebratularis, Lam. 



Teredina personata, Desk. 



Unio Deshayesii, Wat. 



*Cerithium variabile, Desh. 

 *Melama inquinata, Desh. 

 *Melanopsis buccinoides, Fer. 



ancillaroides, Desh. 



Neritina consobrina, Fer. 



globulus, Defr. 



pisiformis, Fer. 



Paludina lenta, Sow. 



Planorbis laevigatus, Desh. 



Those marked with an asterisk abound in both countries. 



Of the remaining twenty-two, the following eight are met with in 

 the Lower Sands of Bracheux or of Rheims : — 



Cardium Plumsteadiense, Sow. 

 Corbula Regulbiensis, Mor. 

 Cyrena intermedia, Mell. 

 Buccinum fissuratum, Desh. 



Calyptraea trochiformis, Lam. 

 Fusus latus, Sow. 



planicostatus, Mell. 



Neritina vicina, Mell. ? 



* Geol. du Departement de la Somme, pp. 49, 33, & 47. 



t There are other indications in M. Graves's monograph, as well as in that 

 of M. Buteux, which would lead me to believe that the term " argile plastique " 

 might be more frequently applied to some of the overlying clays. Op. cit. pp. 

 250, 239, & 244. That the name is not applied, or that they are referred to drift 

 or " remanied " beds, is not sufficient. 



