Ch. XVII.] 



WHITE CHALK. . 



317 



observed in recent coral reefs. It has been quarried to the depth of 

 more than 40 feet, but its thickness is unknown. The imbedded 

 shells are chiefly casts, many of them of univalve mollusca, which arc 

 usually very rare in the white chalk of Europe. Thus, there are two 

 species of Cyprcsa, one of Olivet, two of Mitra, four of the genus 

 Cerithium, six of Fusus, two of Trochus, one Patella, one Emargi- 

 tmla, &c. ; on the whole, more than thirty univalves, spiral or patelli- 

 form. At the same time, some of the accompany- 

 ing bivalve shells, echinoderms, and zoophytes are 

 specifically identical with fossils of the true Creta- 

 ceous series. Among the cephalopoda of Faxoe 

 may be mentioned Baculites Faujasii and Belem- 

 nites mucronatits, shells of the white chalk. The 

 Nautilus Danicus (see fig. 278) is characteristic 

 of this formation ; and it also occurs in France in 

 the calcaire pisolitique of Laversin (Department 

 of Oise). 



The claws and entire skull of a small crab, 

 Brachyurus rugosus (Schlottheim), are scattered | 

 through the Faxoe stone, reminding us of similar | 

 crustaceans enclosed in the rocks of modern coral a 



c 



reefs. Some small portions of this coralline for- - 

 mation consist of white earthy chalk ; it is there- q 

 fore clear that this substance must have been pro- s 

 duced simultaneously — a fact of some importance, I 

 as bearing on the theory of the origin of white -f 

 chalk ; for the decomposition of such corals as we * 

 see at Faxoe is capable, we know, of forming white I 

 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 S 

 Faxoe grew. g 



White chalk (see Tab., p. 314 et seq.).— The g 

 highest beds of chalk in England and France con- 1 

 sist of a pure, white, calcareous mass, usually too 

 soft for a building-stone, but sometimes passing 

 into a more solid state. It consists, almost purely, 

 of carbonate of lime ; the stratification is often 

 obscure, except where rendered distinct by inter- 

 stratified layers of flint, a few inches thick, occa- 

 sionally in continuous beds, but oftener in nodules, 

 and recurring at intervals from two to four feet 

 distant from each other. 



This upper chalk is usually succeeded, in the 

 descending order, by a great mass of white chalk 

 without flints, below which comes the chalk marl, in which there is a 

 slight admixture of argillaceous matter. The united thickness of the 



