PROBLEM I. JVe Lear/? about Earth'' s History froyn Rocks 



527 



Fk;. 476 Plant and animal fossil iinprints found in Fennsylvania. Some of the fern- 

 like plants are iinicb like existing ferns. \\l?at is the fossil at the right F (ward's 



NATURAL SCIENCE ESTABLISHMENT) 



huge numbers of petrifactions. Heavy 

 animals left footprints, but even jellyfish 

 and delicate water plants have left im- 

 prints in the mud which hardened into 

 rock. You can demonstrate how such 

 fossils could have formed by using plas- 

 ter of Paris. See Exercise 6. 



Coal is a rich source of plant fossils. 

 Coal was formed in past ages from for- 

 ests growing in swamps. As the land 

 slowly sank the trees gradually became 

 submerged (covered with water). Thus 

 the leaves, branches, and trunks of the 

 trees were kept from decay. The tre- 

 mendous pressure and heat caused a 

 chemical change that removed almost all 

 the elements except carbon. The almost 

 pure carbon is our coal, and in it are 

 many imprints of the parts of plants. 

 There are often solid leaves and stems 

 of carbon. 



Occasionally fossils have been formed 

 on dry land too when sand, dust, or vol- 

 canic ash was blown over the body of 

 an animal or plant. These materials may 

 harden into rock, thus embedding hard 

 parts of the organism. 



Fig. 477 Dinosaur footprints hardened in stone. 

 From a quarry in West Orange, New Jersey. 



(AMERICAN MUSEUM OF NATURAL HISTORY) 



Other types of fossils. In Los Angeles 

 are pools of tar or asphalt. x\nimals 

 caught in this sticky tar sank and were 

 killed. The flesh gradually disappeared 

 but the tar preserved the skeletons with- 

 out the usual petrifaction. Since they are 

 the remains of bygone ages, they are also 

 called fossils. 



