Mr. De la Beche on the Geologi/ of Part of France. 89 



It will be seen from the preceding observations that, with the exception of 

 quartz rock, the rocks described in this communication nearly correspond in 

 position, structure, and organic remains, with those found in the southern 

 parts of England; of which they are most probably the continuation. The 

 chalk which forms the coast of the department of the Seine Inferieure seems 

 to be a prolongation of that of the Isle of Wight and Dorsetshire. The same 

 maybe said of the green sand ; — which in Devon and Dorset covers in succes- 

 sion oolite, lias, and new red sandstone, the intermediate rocks being want- 

 ing; and which in Calvados rests in succession upon several members of the 

 oolite formation. The oolites and lias of Calvados are probably the same as 

 those of Weymouth, Bridport, and Lyme. The new red sandstone of Calva- 

 dos and La Manche agrees in position, though not always in appearance, with 

 the same rock in Devon. The porphyritic conglomerate of the Bruyere de 

 Crecy and St. Laurent de Condel bear a striking resemblance to that near 

 Exeter ; and the slate of Calvados and La Manche is probably prolonged into 

 Devonshire. 



The incorrectness of the once popular notion that England has been torn 

 from France by some sudden violence, has already been exposed by Mr. W. 

 Phillips * and others. It is much more reasonable to suppose that the bed of 

 the English channel was formed at the same time, and in the same manner, 

 with those differences of level which are observable every where on dry land. 

 The depression that forms the bed of the English channel is very trifling in 

 proportion to its extent of surface ; and is very much exceeded in depth by the 

 large continental lakes. The greatest observed depth of water at the western 

 extremity of the channel, between the Land's End and Ushant, is only seventy 

 fathoms, the distance between the two places being about thirty leagues f ; 

 whereas the Lake of Geneva is nearly double that depth between Lausanne 

 and Evian, the distance between the two latter places being only about three 

 leagues. The greatest depth of water in the narrowest part of the channel, 

 between Dover and Cap Blanc Nez, is thirty fathoms, the distance six leagues ; 

 and between Cap La Hogue and the Isle of Portland, a distance of about sixteen 

 leagues, the depth of water is only forty-five fathoms, or two hundred and 

 seventy feet. 



* In his examination of the cliffs on each side of the Dover Straits. Geol. Trans. Vol. v. 

 + Leagues, thirty to a degree. 



VOL. VI. 



