524 



The Earth mid Its Inhabitants Change unit x 



Fig. 471 (above) What has 

 happened to these layers of 

 sedimentary rock? (geologi- 

 cal SURVEY OF CANADA) 



The strata of sedimentary rock may 

 later be pushed up so that they stand on 

 end. They may be crushed and twisted 

 by the terrific pressure of the earth and 

 the heat which is generated, until their 

 structure is completely changed. This 

 may happen to igneous rocks, too. All 

 such changed rocks are called nietmnor- 

 phic rocks. Marble is a metamorphic 

 rock that was once limestone. Slate is a 

 metamorphic rock that formed from 

 shale; gneiss is a metamorphic rock that 

 formed from granite. To learn more 

 about rocks do Exercise 5. 



Fig. 472 (left) Lava from 

 one of the Hawaiian peaks 

 approaching a fishing vil- 

 lage. These lava flows have 

 been known to extend 50 

 miles, (aeronautical cham- 

 ber OF commerce) 



The earth is constantly changing. 



Sometimes there are sudden changes in 

 the earth as when an earthquake raises a 

 mountain range as much as t^venty feet 

 in a few seconds; or as when during an 

 eruption in the last century the famous 

 volcano Krakatoa blew away a cubic 

 mile of rock, sending up clouds of ashes 

 seventeen miles high. Such changes occur 

 rarely but in the long histor\^ of the 

 earth there must have been many of 

 these changes. Then there are the slow 

 changes that take place when a less ex- 

 plosive volcano pours out masses of mol- 



