FOSSILS, AND HOW THEY ARE FORMED 15 



replacing more or less of the woody tissue and 

 thus really partially changing the wood into 

 stone. 



The very rocks themselves may consist large- 

 ly of fossils ; chalk, for example, is mainly made 

 up of the disintegrated shells of simple marine 

 animals called foraminifers, and the beautiful 

 flint-like " skeletons " of other small creatures 

 termed rj^diolarians, minute as they are, have 

 contributed extensively to the formation of 

 some strata. 



Even after an object has become fossilized, 

 it is far from certain that it will remain in good 

 condition until found, while the chance of its 

 being found at all is exceedingly small. When 

 we remember that it is only here and there 

 that nature has made the contents of the rocks 

 accessible by turning the strata on edge, heav- 

 ing them into cliffs or furrowing them with 

 valleys and canyons, we realize what a vast 

 number of pages of the fossil record must 

 remain not only unread, but unseen. The 

 wonder is, not that we know so little of 

 the history of the past, but that we have 

 learned so much, for not only is nature care- 



