CHAPTER XV 



THE CRUST OF THE EARTH, MAN'S STOREHOUSE 



201. The Formation of the Earth's Crust. We do not 



know what was the beginning of the earth, how it came 

 to have a cold surface 

 and a hot interior, or 

 where the atmosphere 

 and the oceans came 

 from. But we do know 

 that millions of years 

 ago melted rock was 

 squeezed out from the 

 interior of the earth and 

 became solid at the sur- 

 face. The rocks within 

 reach of the water at 

 once began to wear 

 away, as the waves beat 

 upon them. 



Some of the soluble 

 minerals composing the 

 rocks were dissolved, and 



FIG. 88. WATER-WORN ROCKS 



These rocks have been acted upon for 

 unnumbered centuries by air, water, heat, 

 and cold. 1. Name a possible effect of 

 each upon the minerals composing the 

 rocks. 2. Many cracks are exposed; 

 how do they assist in making frag- 

 ments? 



the insoluble minerals 

 formed fragments and 

 grains of different sizes, 

 more or less water- worn. 

 Finally these fragments were distributed in beds or layers by 

 the action of the waves and currents. The spaces between 

 the fragments became filled with a cementing substance ob- 

 tained from the dissolved minerals, and thus the whole 

 mass was consolidated or bound together into new rock. 



177 



