120 MINUTE MARVELS OF NATURE 



marbles is due to the remains of extinct zoophytes ; 

 and fossil corals add to the artistic beauty of other 

 rock structures by their marvellous effects when 



Fig. 77. Section of rock showing fossil corals 



crystallised with other substances. A section of 

 coralline rock is illustrated in Fig. 77, but no 

 photograph can display its beauty of tints. How 



