Ch. XVIL] 



WHITE CHALK. 

 Tig. 249. 



239 



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U'autilus Banicus, Schl.— Faxoe, Denmark. 



The claws and entire skull of a small crab, Brachyu- 

 rus rugosus (Schlottlieim), are scattered tlirongh the 

 Faxoe stone, reminding us of similar crustaceans in- 

 closed in the rocks of modern coral reefs. Some 

 small portions of this coralline formation consist of 

 white earthy chalk ; it is therefore clear that this sub- 

 stance must have been produced simultaneously ; a 

 fact of some importance, as bearing on the theory of 

 the origin of white chalk ; for the decomposition of 

 such corals as we see at Faxoe is capable, w^e know, of 

 forming white mud, undistinguishable from chalk, and 

 which we may suppose to have been dispersed far and 

 wide through the ocean, in which such reefs as that of 

 Faxoe grew. 



White chalk (see Tab. p. 236, et seq.). — The highest 

 beds of chalk in England and France consist of a pure, 

 white, calcareous mass, usually too soft for a building- 

 stone, but sometimes passing into a more soHd state. It 

 consists, almost purely, of carbonate of lime ; the strati- 

 fication is often obscure, except where rendered distinct 

 by interstratified layers of flint, a few inches thick, occa- 

 sionally in continuous beds, but oftener in nodules, and 

 recurring at intervals from 2 to 4 feet distant from 

 each other. 



This upper chalk is usually succeeded, in the descend- 

 ing order, by a great mass of white chalk without flints, 

 below which comes the chalk marl, in which there is a 

 slight admixture of argihaceous matter. The united 

 thickness of the three divisions in the south of England 

 equals, in some places, 1000 feet. 



The annexed section (fig. 250) will show the man- 

 ner in which the white chalk extends from England 

 into France, covered by the tertiary strata described 

 in former chapters, and reposing on lower cretaceous 

 beds. 



