on the Organic Composition of Chalk ami Chalk Marl. 385 



Atlas, and of Arabia from Sinai to Lebanon, thus openintr a 

 large field to organic distribution. " 



5. Many of the chalk-h'ke formations bordering on the Me- 

 diterranean in Sicily, Barbary and Greece, really belong to 

 the period of the European chalk formation, as' proved by 

 their organic contents, although commonly held to be differ- 

 ent from the chalk, and considered as tertiary *. 



6. The chalk beds of the South of Europe, around the ba- 

 sm of the Mediterranean, are distinguished from those of the 

 north and east of Europe by numerous well-preserved chalk 

 animalculesj 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 siliceous shells of Infusoria, and flints 

 are wanting; while in the North of Europe beds of flint al- 

 ternate with the chalk, and marls with Infusoria are wantino-. 

 This exchange of character tends to explain the peculiar re- 

 lation of flint to chalk, indicating that the pulverulent sili- 

 ceous particles of Infusoria have been converted into compact 

 nodules of flint. 



8. It has been lately remarked that the chalk which con- 

 tains flints is deficient in numerous siliceous Infusoria, when 

 compared with the Bilin slaty Tripel or polishing slate {Po- 

 hrschiejer) containing semi- opal ; but this deficiency now dis- 

 appears, and a rich substitute takes its place, the Infusoria in 

 the North of Europe having been employed in the formation 

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

 preserved in the Infusoria marls. 



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

 sand and the Miliolites, which, up to the present day, have 

 been ranged among the Mollusks with the Cephalopods ; but 

 neither ot these are either Cephalopods or Mollusks, nor even 

 Infusoria (as asserted by a late observer) ; but they are Bry- 

 ozoa, 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 course of formation by living Bryozoa, which, 

 though very small, resembling grains of sand, are yet, for the 

 most part, larger than the chalk animalcules, and a large pro- 



* In Sicily, however, there occur many hreccias of chalk, which have 

 sulfercd a subsequent change, and may he referred to tiic tertiary epoch. 



\ 1 hus Ml the white and yellow soft writin;; chalk of the North of Europe 

 the inorganic crystalloid jwrtions sometimes equal or rather exceed in mass 

 the organic remains ; hut in the South of Europe, in Sicily, these organisms 

 with their (ragmcnts arc greatly predominant, consisting," as it .ippears. c\. 

 clusively of well-preserved Polythalamia. u • 



Phil. Mag. S. 3. Vol. IB, No. US. Mai/ 18H. 2 C 



