14 ANIMALS OF THE PAST 



limestone, together with shells of existing spe- 

 cies and fragments of pottery. In a similar way, 

 too, human remains in parts of Florid^ have, 

 through the infiltration of water charged with 

 iron, become partially converted into limonite 

 iron ore ; and yet we know that these bones 

 have been buried within quite recent times. 



Sometimes we hear of springs or waters that 

 " turn things into stone," but these tales are 

 quite incorrect. Waters there are, like the 

 celebrated hot springs of Auvergne, France, 

 containing so much carbonate of Ume in solu- 

 tion that it is readily deposited on objects 

 placed therein, coating them more or less 

 thickly, according to the length of time they 

 are allowed to remain. This, however, is mere- 

 ly an encrustation, not extending into the ob- 

 jects. In a similar way the precipitation of 

 solid material from waters of this description 

 forms the porous rock known as tufa, and this 

 often encloses moss, twigs, and other substances 

 that are in no way to be classed with fossils. 



But some streams, flowing over limestone 

 rocks, take up considerable carbonate of lime, 

 and this may be deposited in water-soaked logs, 



