THE CRUST OF THE EARTH 189 



uids, but they left imprints from which the character of 

 the organism can be determined. Fossil leaves are often 

 found on layers of slate in beds of coal. 



Only fossils of the simplest animals and plants are found 

 in the lower, older rocks, while fossils of more complex ani- 

 mals and plants appear in the later rocks. The study of life 

 has shown some of the changes that have occurred since few 

 and simple forms of plants and animals began to live. Long 

 ages must have elapsed during the period when the later, 

 more complex forms of life were developing from the simple 



forms. 



EXERCISES 



1. What kinds of matter form the sediment of a river? Where does 

 it come from? 



2. W T hy are layers of shale and sandstone found with coal beds? 



3. What does conglomerate rock tell of its origin? Why? 



4. Why does the folding of layers of rock produce great heat? 



5. What changes in rock does the heat of folding cause? 



6. (a) Name the metamorphic rocks made from sandstone and shale. 

 (6) From limestone. 



7. Give a reason why more coal is mined in Pennsylvania than in 

 Indiana. 



8. Iron ore from the vicinity of Lake Superior is sent to Pennsylva- 

 nia for reduction. Give a reason why it is sent so far for reduction. 



9. Name three crystalline rocks consisting of the same minerals. 



