HOW WERE THE ROCKS FORMED? 219 



will find that your knife blade will scratch some and not 

 others. You can scratch your knife blade with quartz, 

 while the blade will easily scratch such a mineral as talc 

 or rock salt. Minerals, evidently, differ in hardness. 

 They also differ in other respects, such as color, chemical 

 composition, the kind of crystals they form, and other 

 ways. Because of these differences, the rocks out of 

 which they are made also greatly differ. Some are hard, 

 others relatively soft ; some strong, others brittle. 



Rocks Change to Soil. If what has just been said is 

 true, then the change from rock to soil must go on much 

 faster in some rocks than in others. Soils also vary in differ- 

 ent places, depending on the kind of rock they are made 

 from. Quartz, for example, is harder than feldspar. 

 When granite breaks down to form soil, the quartz par- 

 ticles, being harder, grind the rest of the rock to fine 

 powder, while they remain as grains of pure quartz. 



SELF-TESTING EXERCISE 



Select from the following list those words which best fill the blank spaces 

 in the sentences below and arrange the words in proper numerical order. 

 A word may be used more than once. 



hard heat sedimentary ice 



soft cold durability clouds 



softer igneous loose metamorphic 



rock cut quartz scratch 



solid molten soil melting 



mineral vaporized water chemical 



mud solidified air layers 



Granite is an example of an (1) rock which formed from a 



(2) condition. Sandstone is a (3) rock and was once 



(4) particles which eventually were brought into (5) , 



probably through the action of (6) After being buried deep 



in the earth loose material may under the action of (7) , 



pressure, and cementing by (8) action be changed into solid 



(9) Both sedimentary and (10) rocks may undergo a 



partial (11) and be changed greatly in form. This class of rock 



resulting is called (12) Most rocks are made up of two or more 



