PRESTWICH BRITISH AND FOREIGN TERTIARIES. 231 



denuded for many miles to the south-eastward of the hills of St. 

 Marguerite ; and at the next mass of the Lower Tertiaries, in the 

 neighbourhood of Gisors and Chaumont, we could find no traces of 

 that deposit. 



If, however, instead of following in this direction towards Paris, 

 we take a northerly direction through Cassel, Lille, and Tournay, we 

 shall find every reason to believe that London Clay extends through- 

 out the greater part of French Flanders and Belgium. I have before 

 noticed the occurrence of this clay beneath Calais*. I have since seen 

 it at the hills adjoining Watten, near St. Omer, where it rises 220 

 feet above the plain, with its ordinary aspect and mineral characters 

 just as well marked as at the Norwood hills, or at Primrose Hill, Its 

 thickness also at Watten cannot be less than 300 to 350 feet. 



M. Dumont and Sir Charles Lyell have shown that the Ypresien 

 Clay (London Clay) has a considerable range in Belgium. The 

 observations of Sir Charles Lyell are particularly pertinent to this 

 point, for he shows by a well-section at the Railway Station at the foot 

 of Cassel Hill, where many of the beds of the Paris Tertiaries are 

 well exhibited, that the London Clay, perfectly well characterized by 

 its mineral character (which he resembles to that of Highgate) and 

 with Septaria, is there more than 291 feet thick. If to this we add 

 118 feet for the elevation of the London Clay above the spot where 

 the well was sunk, it will give a thickness of more than 400 feet to 

 the London Clay in that district, proving it to have a development 

 probably equal to that which it possesses in the neighbourhood of 

 London. Further, Sir Charles mentions that it is only about 150 

 feet thick near Lille, showing, therefore, a gradual thinning-out as 

 it ranges southward into France (see PL VIII. Sect. 2). No fossils f 



* M. Meugy mentions {op. cit. p. 154) many other well-sections proving the same 

 fact. One at Dunkirk was carried by boring through 118 feet of sands (called sea- 

 sand), and then 266 feet into the London Clay; another at Hazebrouck reached 

 the base of the London Clay (here covered only by a few feet of drift), at a depth 

 of 328 feet. His work contains many other sections proving the importance of the 

 Glaise Ypresien (London Clay) in French Flanders. 



t This has been urged as a serious objection by some French geologists, who, 

 however, have not hesitated to refer these clays to a large development of the 

 clays of the " Argile Plastique," — a correlation which would be attended not only 

 by the same difficulty to which they here object, but would further want all the 

 analogies which in the other case we possess. It must also not be forgotten that in 

 England the London Clay is frequently non-fossiliferous; that even in cliff-sections, 

 as for example at Sheppey or Heme Bay, it requires a careful search to discover 

 any fossils in the clay itself. As the fossils also are more abundant in particular 

 zones, it is necessary to attend daily during the sinking of a well, as large portions 

 of the clay contain no fossils, whilst in other portions they may be plentiful. I 

 doubt whether there have been opportunities to examine the London Clay in the 

 north of France and Belgium with sufficient care. 



Since writing this paper, I find that M, Meugy notices a single instance in 

 which fossils were found in beds 55 feet thick, which he refers to the " Glaise 

 Ypresien." He states that they were tolerably abundant, and consisted of species 

 of Turritella, Venericardia, Cardium, Lucina, Ostrea, Pleurotoma, and tjie Num- 

 mulites planulatus, and in some underlying sands Ostrea, Peeten, and Dentalium. 

 I should almost fear some mistake here. This is certainly not a group of fossils 

 found in the London Clay in this country. The section, however, is not sufficiently 

 definite to feel sure as to the position of the beds. Op. cit. p. 143. 



