206 



ROCKS AND SOILS 



not state exactly the nature 

 of all the forces at work 

 producing these changes. It 

 is believed that heavier ma- 

 terials move toward the 

 center of the earth and push 

 lighter materials upward. 

 Materials under the oceans 

 are slightly heavier than 

 those of land areas. The 

 difference in pressure would 

 tend to elevate the con- 

 tinents and deepen the 

 oceans. The fact that some 

 mountain systems, such as 

 the Rockies and the Appa- 

 lachians in North America, 

 are near the coast line tends 

 to support'this belief. 



Earthquakes. Sometimes 

 movements of the earth's 

 crust are accompanied by 

 sudden slipping of rock. Vi- 

 brations are then set up that travel through the earth. 

 These vibrations are called earthquakes. Generally 

 these vibrations produce only a slight trembling of 

 the earth, but occasionally the vibratory motion is so 

 intense as to cause buildings to tumble down. An en- 

 tire city may be wrecked in a few minutes. 



William Thompson 

 FIG. 332. CRACK OR FISSURE 

 PRODUCED BY EARTHQUAKE 



Earthquakes are recorded and studied by means of 

 an instrument called a seismograph. This instrument 

 consists of a pendulum (a heavy weight hung on a 

 wire) with a pointer on the end of it that hangs mo- 

 tionless except when the earth is vibrating. The in- 

 strument is very sensitive, and during the tremors of 

 an earthquake the pointer swings back and forth, mak- 

 ing sharp lines on a piece of paper covered with lamp 

 black. The smoked paper is on a rotating drum that 

 is turned automatically by a clock, making it possible 

 to tell from the marks left on the paper the exact time 

 of day the earthquake took place. 



Exercise. From your study, cite evidence in support 

 of each of the following general statements. 1 



1. The surface of the earth is ever changing. 



2. Forces tending to wear away the earth's surface 

 are at work. 



3. Scientists have learned some facts about the in- 

 ternal structure of the earth. 



REFERENCES FOR FURTHER STUDY 

 Texts 



Caldwell and Curtis, Science for Today, Chap. 19 

 Clement, Collister, and Thurston, Our Surroundings, Chap. 21 

 Hunter and Whitman, My Own Science Problems, Unit 9; 



Science in Our Social Life, Unit 10 



Lake, Harley, and Welton, Exploring the World of Science, 

 Chap. 13 



1 See workbook, p. 75. 



."XTpStart of 2nd Preliminary Tremors 

 ^^ ~il:53'.-45vP.M. Nov. to'. 1922 ': 



3.30A.M. 

 'V V*SM- 



12 00 Mldn 



'Section 2 



Courtesy American Museum of Natural History 



FIG. 333. SEISMOGRAPH RECORD OF AN EARTHQUAKE IN CHILE 



