CHAP. XII.] ICENIAN PERIOD. 239 



Sheppey and Essex they doubtless rested on some member 

 of the Bagshot group, and thence their base-line passed 

 across the outcrops of the Lower Eocene and Cretaceous 

 strata till it rested on the Wealden beds over the central 

 axis of the Wealden area (see fig. 6). 



Outliers of similar ferruginous sands occur also at inter- 

 vals in France and in southern Belgium, the level of their 

 base-line sinking lower and lower to the eastward. Thus, 

 at Cape Blanc-Nez, they are about 500 feet above the sea ; 

 at Cassel, near Ypres, their level is 470 feet ; above 

 Itenaix it is 445 ; at Gramont 380 ; on the heights above 

 Brussels it is only 245, and near Diest, though the level 

 varies considerably, their base nowhere rises above a height 

 of 200 feet. 1 Northward the base-line sinks still more 

 rapidly, and is less than 20 feet above the sea near Anvers 

 (Antwerp). Yet throughout this range the sands are simi- 

 lar in constitution and contents, proving that the present 

 differences of level are due to subsequent differential move- 

 ment of the land. Around Diest and Tessenderloo they 

 cover a large area, and attain a thickness of nearly 100 feet ; 

 at Antwerp they contain many fossils, and form the zone 

 of Isocardia cor, passing beneath newer Pliocene beds. 

 Northwards, as they pass beneath Holland, their thickness 

 increases, for at Utrecht a deep boring passed through the 

 overlying beds and 410 feet of Diestien Sand without 

 reaching their base, though carried to a depth of 1,208 feet. 

 The range of these beds in Belgium and the north of France 

 is shown in fig. 7, the eastern portion of which is reduced 

 from Vandenbroeck's map. 



Returning to England, fine yellow sands, sandstones, 

 and sandy marls of somewhat similar character form the 

 lower division of what is known as the Coralline Crag in 

 Suffolk, and as nearly all the Lenham species occur also in 



1 Vandenbroeck, " Bull. Soc. Beige Geol.," &c., tome i. p. 51, and 

 map. 



