ON THE ORGANIC COMPOSITION OF CHALK. 87 



to the Atlas, and of Arabia from Sinai to Lebanon, thus opening a large 

 field to organic distribution. 



5. Many of the chalk-like formations bordering on the Mediterranean 

 in Sicily, Barbary, and Greece, really belong to the period of the Euro- 

 pean chalk-formation, as proved by their organic contents, although 

 commonly held to be different from the chalk, and considered as ter- 

 tiary. * 



6. The chalk-beds of the south of Europe, around the basin of the 

 Mediterranean, are distinguished from those of the north and east of 

 Europe, by numerous well-preserved chalk animalcules, and less 

 numerous inorganic lamina? ; while in the north and east of Europe 

 these relations are reversed, f 



7. In the south of Europe the beds of marl which alternate with the 

 chalk, consist of silicious shells of Infusoria, and flints are wanting ; 

 while in the north of Europe beds of flint alternate with the chalk, and 

 marls with Infusoria are wanting. This exchange of character tends to 

 explain the peculiar relation of flint to chalk, indicating that the pulve- 

 rulent silicious particles of Infusoria have been converted into compact 

 nodules of flint. 



8. It has been lately remarked, that the chalk which contains flint is 

 deficient in numerous silicious Infusoria, when compared with the Bilin 

 slaty Tripel, or polishing slate (Polirschiefer}, containing semi-opal ; 

 but this deficiency now disappears, and a rich substitute takes its place, 

 the Infusoria in the north of Europe having been employed in the for- 

 mation of flints ; while in the south, remaining unchanged, they are 

 preserved in the Infusoria marls. 



9. The chalk animalcules most resemble those of the sea-sand and 

 the Miliolites, which, up to the present day, have been ranged among 

 the Molluscs with the Cephalopods ; but neither of these are either 

 Cephalopods or Molluscs, nor even Infusoria, as asserted by a late ob- 

 server ; but they are Bryozoa, animals of Moss-corals, which are most 

 nearly related to Flustra and Eschara. 



10. The sea downs of some, and probably of most coasts, are still in 



* In Sicily, however, there occur many breccias of chalk, which have suffered a 

 subsequent change, and may be referred to the tertiary epoch. 



f Thus in the white and yellow soft writing chalk of the north of Europe, the in- 

 organic crystalloid portions sometimes equal, or rather exceed in mass the organic 

 remains ; but in the south of Europe, in Sicily, these organisms, with their frag- 

 ments, are greatly predominant, consisting, as it appears, exclusively ot well pre- 

 served Polythalamia. 



