1852.] LYELL — BELGIAN TERTIARY FORMATIONS. 283 



In some districts near the sea it is concealed beneath a superficial 

 covering of stiff tenacious clay of fluviatile origin, provincially called 

 ** Polders/' full of recent land and freshwater shells, and resembling 

 the dark-coloured clay now deposited by the Scheldt during its 

 annual inundations. 



In the absence, therefore, of natural sections, it is not always 

 possible to obtain good proofs of the relative position of particular 

 masses of sands and clay occurring here and there at the surface, 

 although the same have appeared to MM. Nyst and De Wael to 

 contain internal or palaeontological evidence of being of different ages. 

 This conclusion has been deduced principally from the greater or less 

 agreement of the shells with living species, especially with those now 

 existing in the neighbouring sea ; — a subject to which I shall refer 

 more fully in the sequel. 



1. Yellow Crag. Upper Antwerp Crag. 



At Calloo, one of the spots most fertile in fossil shells, situated two 

 leagues N.W. of Antwerp (see Map, PL XVII. fig. 1) on the left 

 bank of the Scheldt, M. de Wael observed the following section of 

 strata referred by him to the upper or yellow crag : — 



Thickness, 

 feet, inches. 



1. Mud or "polder" 1 6 



2. Clay and loam 3 6 



3. Yellow sand 1 



4. Yellow shelly sand 5 



11 



The following is a list of shells from this spot in the collection of 

 M. de Wael, to which, with the assistance of Messrs. Searles Wood 

 and Morris, I have added two columns showing the proportion of 

 Antwerp species common to the Red and Coralline Crag of Suffolk, 

 and another column to indicate which species are still living. In 

 identifying shells, enumerated by M. de Wael in his MS. lists, of 

 which I do not possess specimens from Calloo, we have availed our- 

 selves of M. Nyst's figures and descriptions ; but, if any doubt existed 

 as to the species meant, it has been omitted altogether. 



The column specifying the rarity or abundance of the shells is fur- 

 nished by M. de Wael, and may be useful in determining the depth 

 of the sea in which the strata were formed. Where there is a blank 

 in that column, the species are neither common nor rare, although 

 some of them may be very rare in collections owing to their fragility. 



