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 
fiint-like “skeletons” of other small creatures 
termed radiolarians, 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- 
