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 



