CHAPTER XIII. 



Crag of Norfolk and Suffolk Shown by its fossil contents to belong to the older 

 Pliocene period Heterogeneous in its composition Superincumbent lacustrine 

 deposits Relative position of the crag Forms of stratification Strata com- 

 posed of groups of oblique layers Cause of this arrangement Dislocations in 

 the crag produced by subterranean movements Protruded masses of chalk 

 Passage of marine crag into alluvium Recent shells in a deposit at Sheppey, 

 Ramsgate, and Brighton. 



CRAG OF NORFOLK AND SUFFOLK. 



THE older Pliocene strata, described in the last chapter, are all 

 situated in countries bordering the Mediterranean^ but we 

 shall now consider a group in our own island, which belongs to 

 the same era. We have already alluded to this deposit under 

 the provincial name of crag *, and pointed out its superposition 

 to the London clay, a tertiary formation of much higher anti- 

 quity f. The crag is chiefly developed in the eastern parts of 

 Norfolk and Suffolk, from whence it extends into Essex. 



Its relative age. A collection of the shells of the ' crag' 

 beds, which I formed in 1829, together with a much larger 

 number sent me by my friend, Mr. Mantell of Lewes., were 

 carefully examined by M. Deshayes, and compared to the 

 tertiary species in his cabinet. This comparison gave the 

 following result : out of 111 species, 66 were extinct or un- 

 known, and 45 recent, the last, with one exception ( Valuta 

 Lamberti, Sow.), being now inhabitants of the German ocean. 

 Such being the proportion of recent and extinct species,, we may 

 conclude, according to the rules before laid down J, that the 

 crag belongs to the older Pliocene period. 



Mineral composition. So heterogeneous is this deposit in 

 mineral character, that we can scarcely convey any correct no- 

 tions of its appearance, without describing the beds separately 

 in the different localities where they occur. In general, they 

 * Chap. ii. p. 19. f See above, Diagram No. 4, p. 21. J Page 54. 



