162 A YEAR IN SCIENCE 



depth. As the heat recedes towards the center of the 

 earth by cooling, the waters follow. In the very 

 remote future the earth will have cooled to a depth 

 permitting the absorption of all the water and atmo- 

 sphere now on its surface. Such is considered to be 

 the condition of the moon now. 



Continents are great bodies of land surrounded by 

 water. The average height of the land above sea is 

 less than one-half mile. While continents have under- 

 gone many changes in the past, they have practically 

 always occupied their present position. 



Mountains, plateaus, and plains. Mountains, the 

 glory of the earth, the culminating places of scenic 

 grandeur and beauty, are the most conspicuous land 

 forms. Most mountain systems are located on the 

 sides of continents adjacent to the ocean ; furthermore, 

 the highest mountains border the largest oceans. The 

 birthplaces of mountains are marginal sea bottoms. 

 As the interior of the earth loses heat it contracts. 

 But, as the crust of the earth is already cool, it is 

 more or less rigid ; hence, to accommodate itself to 

 the shrinking interior, it must fold. This folding 

 naturally takes place along lines of weakness in the 

 earth's crust. Marginal sea bottoms are lines of weak- 

 ness. Here for ages have accumulated enormous 

 deposits of sediment. As the sediment accumulates, 

 the interior heat rises toward the surface, and in the 

 presence of water included in the sediment, produces, 

 a semi-fused condition, This condition, of course, pro-. 



