PROBLEM I. We Learn about Earth's History fro?n Rocks 531 



Questions 



1. State one theor\^ of the origin of the earth. Describe the earth in the 

 early stages. What is igneous rock? How did soil gradually form? 

 What does a geologist study? 



2. Define erosion. Define weathering. What effect has erosion on the 



height of mountain ranges? 



3. Explain what becomes of the rock and soil carried bv streams. 



4. How is sedimentary rock formed? How do strata form? How do 

 metamorphic rocks differ from sedimentary? Give three examples 

 of metamorphic rocks. 



5. Give examples of sudden earth changes and of slow changes. 



6. What is now considered to be the most accurate way of estimating 

 the age of the earth? How old is the earth supposed to be? 



7. How long a stretch of time is an era? Into what are eras divided? 

 How do we learn about the early inhabitants of the earth? 



8. Explain how fossils form by petrifaction. What kinds of fossil im- 

 prints have been found? Why is coal a rich source of plant fossils? 



9. Describe three ways besides petrifaction by which remains of animals 

 or plants of past ages can be preserved. Give a full description of the 

 term fossil. 



10. How may deeply buried fossils be discovered? Why are fossils fre- 

 quently found at or near the surface of the earth? 



11. What important facts about the former inhabitants of the earth have 

 we learned from a study of fossils? 



Exercises 



1. Demonstration. The slow action of carbonic acid on limestone can 

 be shown, speeded up enormously, by using a stronger acid. Pour dilute 

 hydrochloric acid on marble chips. What becomes of the marble? 



2. You can show the force of germinating seeds by planting some 

 soaked pea and corn seeds in moist sawdust or sand in a tumbler. Cover 

 the surface of the sawdust or sand with paraffin, making holes in several 

 places (where no seed has been planted) so as to allow the air to reach 

 the germinating seeds. Can you devise any other experiment to show the 

 force of germinating plants? 



3. Demo?istratio?i by teacher or pupil, (a) Prepare two mounds of 

 earth (preferably clay), each in a box. Cover one mound with sod. Leave 

 the other unsodded. Using a watering can, pour equal amounts of water 

 at the same rate on each mound. What do you demonstrate by this 

 means? (b) When doing the demonstration in (a), collect, in separate 

 containers, the water which has run off each mound. Allow the sedi- 

 ment to settle. Does the kind of country drained by a river make a differ- 

 ence in the speed with which a delta forms? Explain. • 



