HOW WERE THE ROCKS FORMED? 



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rock. Perhaps a million years later this part of the earth 

 rose, the surface layers were worn off, and this layer of 

 material is back at the earth's surface once more, but 

 now solid sandstone and not loose particles. Such rocks 

 are called sedimentary. 



Another kind of rock seems to be in layers, but these 

 layers are greatly curved or folded, like the rock shown 

 in the picture. These look as if they might have been 

 made like sedimentary rocks and then pressed together 

 by some great force. Possibly they might have been the 

 igneous rocks partly remeltetl, and pressure caused a 

 movement so that particles appear in bands somewhat 

 resembling layers. People who have made a study of 

 rocks believe both of these processes have been in action 

 and have caused these rocks to be changed from the 

 original condition. They are called metamorphic rocks. 

 Examples of such rocks are gneiss, marble, and slate. 



Rocks and Minerals. Geologists call the material out 

 of which the solid part of the earth is formed rock. But 

 if you look at some rocks carefully, you will see they are 



Geographical Survey, G. K. Gilbert Negative 



Geographical Survey, T. N. Dale Negative 



Sedimentary and metamorphic rock. How does the right-hand picture differ 

 from the left-hand one ? What seems to have happened to the metamorphic rock ? 



