FOSSILS, AND HOW THEY ARE FORMED 3 
sions of their leaves left on the soft mud or 
smooth sand that later on hardened into endur- 
ing stone. Such, too, are the trails of creeping 
and crawling things, casts of the burrows of 
worms and the many footprints of the reptiles, 
great and small, that crept along the shore or 
stalked beside the waters of the ancient seas. 
The creatures themselves have passed away, 
their massive bones even are lost, but the prints 
of their feet are as plain to-day as when they 
were first made. 
Many a crustacean, too, is known solely or 
mostly by the cast of its shell, the hard parts 
having completely vanished, and the existence 
of birds in some formations is revealed merely 
by the casts of their eggs; and these natural 
casts must be included in the category of 
fossils. } 
Impressions of vertebrates may, indeed, be 
almost as good as actual skeletons, as in the 
case of some fishes, where the fine mud in 
which they were buried has become changed 
to a rock, rivalling porcelain in texture; the 
bones have either dissolved away or shattered 
into dust at the splitting of the rock, but the 
