14 ANIMALS OF THE PAST 
limestone, together with shells of existing spe- 
cies and fragments of pottery. Ina similar way, 
too, human remains in parts of Florida 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 lime 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, 
