Wealden District and the Bas Boulonnais. 27 



which after a gradual southerly declination the clay passes entirely under the 

 coral rag. 



Coral Rag. — This formation is found to basset regularly in a band of greater or 

 less width on the west of each of the hills above described as having its escarpment 

 formed principally by the Oxford clay. From the point at which that formation 

 entirely disappears (as just stated) beneath the coral rag, the latter occupies the 

 valley of the Liane to a considerable elevation on both sides of the river, till it ar- 

 rives within about two miles of Boulogne. It passes also under the eminence on 

 which Samer, on the Paris road, is situated, beyond which it is covered up by the 

 lower greensand. 



Kimmeridge Clay. — This formation succeeds in ascending order. North of Amble- 

 teuse it occupies a narrow tract along the coast. On the south of the river it extends 

 from the coast by Maninghen to the east of Hesdres, over the high ground which 

 intervenes between the Ambleteuse and the Wimereux rivers. And on the south 

 of the latter it extends in like manner from the coast into the For^t de Boulogne, 

 and occupies the heights immediately to the north of that part of the valley of the 

 Liane along which runs the Paris road, the sides of the valley itself being occupied 

 by the coral rag, as before stated. The most eastern point to which it proceeds is 

 Samer. The insulated hill on which that place is situated is capped by some of its 

 lower beds, distinctly characterized by the Gryphaa virgula , and resting on the coral 

 rag which forms the base of the hill (as already mentioned), and extends to the 

 valley of the Liane. On the south of the Liane it extends from the coast in a similar 

 manner, till the coral rag emerges from beneath it, or it becomes concealed by the 

 unconformable covering of lower greensand. 



This formation is best exhibited in the coast section immediately to the north of 

 Boulogne. The lower division consists of a species of blue marl, with not unfrequent 

 hard stony bands, and is for the most part filled with the remains of the Gryphcsa 

 virgula. The upper portion consists of a similar blue marl, frequently somewhat 

 shaly, and containing the Gryphcsa virgula in much smaller quantity. It contains 

 however in considerable numbers a Gryphcea which appears to me a distinct 

 species, and has not, I think, been yet named. This portion is also characterized 

 by the Ostrea deltoidea, of which I never observed a specimen in the lower portion 

 of the formation. These two parts are separated by thick continuous bands of 

 sandstone frequently characterized by large TrigonicB (Diagrams Nos. 27, 28, 30.). 

 They descend to the level of the sea at La Creche (Diagram No. 28.), and furnish 

 the enormous blocks on the beach at that point. It is by means of these distinct 

 divisions of this formation that we are enabled to make out the geological structure, 

 where most comphcated, with a degree of accuracy which would otherwise be un- 

 attainable. 



E 2 



