EXPANSION AND CONTRACTION 



and becomes too large for 

 the interior, and crumbling 

 and splitting result from 

 the strain. With the sud- 

 den fall of temperature in 

 the late afternoon and 

 night, the surface of the 

 rock becomes greatly 

 'chilled and colder than the 

 rock beneath ; the surface 

 rock therefore contracts 

 and shrinks more than the 

 underlying rock," and again 

 crumbling results (Fig. 

 6). 



On bare mountains, the 



FlG. 6. Splitting and crumbling of rock 

 caused by alternating heat and cold. 



heating and cooling effects of the sun are very striking (Fig. 7); 



the surface of many a 

 mountain peak is cov- 

 ered with cracked rock 

 so insecure that a touch 

 or step will dislodge the 

 fragments and start 

 them down the moun- 

 tain slope. The lower 

 levels of mountains are 

 frequently buried sev- 

 eral feet under debris 

 which has been formed 

 in this way from higher 

 peaks, and which has 

 slowly accumulated at 



FIG. 7. Debris formed from crumbled rock. the lower levels. 



