72 



THE EAKTH. 



the roe of a fish, it consists of small rounded particles of 

 limestone set in a cement of the same substance worn 

 down fine. 



These strata furnish a great many organic remains, 

 especially the shells of the conchiferous mollusca and cepha- 

 lopods, as Ammonites (fig. 22), Belemnites, Nautili, and 



PIG. 22. AMMONITE (Henlyi). 



FIG. 23. PENTACBINITE. 



Pentacrinites (fig. 23), of which a great many varieties are 

 found, also the remains of gigantic reptiles, as the Ichthyo- 

 saurus (fig. 24), Plesiosaurus (fig. 25), and others. New 

 forms of animal existence seem to have been created in this 

 period in great abundance, and the waters of the earth 

 once again became the theatre of deposition for the 

 shells and polypidomes of zoophytes and molluscs, which 

 swarmed them in myriads, and another great group of rocks 

 began to be formed, namely, the chalk or " Cretaceous" system, 

 which form the cliffs and downs of our south coasts, and strata 

 of great extent in nearly every part of the world ; it differs 

 from limestone only in not being so hard, which is supposed to 

 arise from its not having undergone the changes caused by 

 heat and pressure. 



