PRESTWICH BRITISH AND FOREIGN TERTIARIES. 233 



According to the description of Sir Charles Lyell, I should estimate 

 these beds not to be together less than 32, and not more than 50 feet 

 thick *. He enumerates only thirty-seven named species of fossils ; 

 of these, thirty-one are found also in the " Calcaire grossier." 



I should be inclined to consider these Cassel beds to represent 

 both the '* Calcaire grossier " and " Glauconie grossiere," or rather, 

 more especially as resulting from a development of the latter and a 

 thinning of the former. At Brussels, the fauna of these beds is 

 rather richer. Sir Charles mentioning forty-five named species f. He 

 also makes the series about 100 feet thick. I should, however, be 

 inclined, on the physical characters he describes, to place the lower 

 40 or even 70 feet with the next underlying series. 



Sir Charles does not give an exact measurement of the lower beds 

 at Cassel, but if we take the total thickness of the beds above 

 described, it will give in round numbers from 140 to 160 feet. Now 

 M. Meugy states that the " Glaise Ypresien " (London Clay) rises to 

 a height at the base of the hill of 247 feet, which would leave 100 to 

 120 feet as the thickness of the siliceous sands which Sir Charles 

 describes as underlying the zone of the Nummulites IcBvigatus and 

 associated beds J. 



In these lower sands, which are referred by M. Dumont and Sir 

 Charles to the " Sables Ypresiens," the Nummulites planulatus has 

 not been found; but it is met with in beds holding the same position 

 between this spot and Courtray, whilst still further eastward this 

 Foraminifer is abundant. The sands beneath this nummulitic zone 

 M. d'Archiac refers to his "Sables divers §." 



We have thus had established at the hill of Cassel, by M. Elie de 

 Beaumont and M. d'Archiac, a succession of strata corresponding in 

 all these central beds with those of the Soissonnais in the Paris district ; 

 whilst the later researches of M. Dumont and Sir Charles Lyell show 

 that that series is underlaid by the London Clay. That the zone of 

 the Cerithium giganteum and Nummulites Icevigatus at Cassel repre- 

 sents the Calcaire grossier on one side, and on the other is correlated 

 with the '^ Systeme Bruxellien " of Belgium, I take for granted upon 

 the authority of these several eminent geologists. The identification, 

 however, of the zones of the *'Lits Coquilliers" and "Sables divers" 

 is attended with more uncertainty; for, although a few fossils, position, 

 and mineral structure all coincide in exhibiting a close analogy, yet it 

 must be admitted that the evidence of organic remains is of itself not 

 very strong. Of the Cassel fossils belonging to these beds, we have 

 no positive list. It is possible, however, that the one given by Sir 



* Op. cit. p. 324. 



t M. Omalius d'Halloy quotes, on the authority of M. Nyst, 95 species of Mol- 

 luscs from the "Sables Calcariferes de Bruxelles " (Systeme Bruxellien, Dumont), 

 but I do not feel quite sure whether his division is exactly the same as that of 

 Sir Charles Lyell, — whether it does not include the N. planulatus zone {Abr. de 

 Geol. p. 579). 



X Including the beds corresponding, according to M. Meugy, with the " Sy- 

 steme Paniselien" of M. Dumont, this portion of the series would be 124 feet thick 

 {op. cit. p. 168), 



§ Bull. See. Geol. de Fr. vol. x. p. 182, 1839 (there termed "Sables inferieurs"). 



