182 LYELL'S ELEMEiNTS OF GEOLOGY. 



Fossils of the Chalk. 



I sliall now speak first of the chalk, 

 its fossils, and probable origin ; and then 

 say something of the green-sand ; after 

 which I shall point out the probable rela- 

 tions of the chalk and green-sand to each 

 other. 



White Chalk. — The white chalk used 

 in writing consists almost purely of car- 

 bonate of lime. Although usually soft, 

 this substance passes in some districts by 

 a gradual change into a solid stone used 

 for building. The stratification is often 

 obscure, except where rendered distinct 

 by alternating layers of flint. These 

 layers are from two to four feet distant 

 from each other, and from three to six 

 inches in thickness, occasionally in con- 

 tinuous beds, but more frequently in 

 nodules. 



The annexed figures represent some 

 few of the fossil shells which are abun 

 dant in the white chalk, and these alone 

 are sufficient to prove its marine origin. 

 Some of them, such as the Terebratulee, 

 (see Figs. 148. 150, 151, 152.) are 

 known to live at the bottom of the sea, 

 where the water is tranquil, and of some 

 depth. The Crania and Catillus (Figs. 

 145. and 144.) may be pointed out as 

 forms which, so far as our present infor- 

 mation extends, became extinct at the 

 close of the cretaceous period, and are 

 therefore never met with in any tertiary 

 stratum, or in a living state. Among 

 other forms, equally conspicuous among 

 the fossil mollusca of the cretaceous 

 group, and foreign to the tertiary and 

 recent periods, may be mentioned the 

 Belemnite, x\mmonite, Baculite, and Tur- 

 rilite of the family Cephalopoda, to which 

 the living cuttle-fish and nautilus belong. 

 One of these, the Belemnite, like the bone of the common 

 cuttle-fish, was an internal shell. Besides these there are other 



fi0 



fossils in the chalk, such as sea-urchins, corals, and spongesj 



