THE NEW EVOLUTION ^^ 



hand very many things which are by no means so 

 evident elsewhere. 



The most important thing we learn is that rocks, 

 no matter how solid they may seem, are far from inde- 

 structible. The rocks of the bare mountain tops are 

 always cracked and broken. The expansion caused 

 by the sun's heat and the contraction caused by the 

 cold of night or of the winter and the freezing and 

 thawing of such water as penetrates the fissures are 

 continually chipping off larger or smaller bits from 

 their exposed surfaces. Besides this, the various 

 minerals which compose the rocks are all more or less 

 soluble in water so that part of the substance of the 

 rocks is continually being washed away. 



The bits chipped off fall down the mountain side 

 and gradually are reduced to smaller and smaller 

 fragments. The substances dissolved are partly held 

 in the water in the soils, and partly are carried by the 

 rivers to the sea. 



This process of rock destruction in most mountain- 

 ous regions is hastened by earthquakes which by 

 shaking the fragments down into the valleys expose 

 new surfaces to the destructive forces. 



In many places volcanoes are continually, or from 

 time to time, bringing to the surface great masses of 

 rock in the form of lava or of dust or ash and together 

 with this water vapor and other gases of various sorts 

 which add to the supply of substances available for 

 the support of life on the earth's surface. 



Rocks appear to us as solid, unyielding, and more 

 or less unattractive objects. But they contain. 



