216 THE BEAUTIES OF NATURE chap. 



with ridges. Or again, if we place some sheets 

 of paper between two weights on a table, and 

 then bring the weights nearer together, the 

 paper will be crumpled up. 



In the same way let us take a section of 

 the earth's surface AB (Fig. 17), and suppose 

 that, by the gradual cooling and consequent 

 contraction of the mass, AB sinks to A'B', 



a:' 



a" 



Kg. 17. — Adapted from Ball's p.iper " On the Formation of Alpine Valleys 

 and Lakes," Lond. and Ed. Phil. Mag. 1863, p. 96. 



then to A"B", and finally to A'"B"'. Of 



course if the cooling of the surface and of the 

 deeper portion were the same, then the strata 

 between A and B would themselves contract, 

 and might consequently still form a regular 

 curve between A'" and B'". As a matter of 

 fact, however, the strata at the surface of our 

 globe have long since approached a constant 

 temperature. Under these circumstances 

 there would be no contraction of the strata 

 between A and B corresponding to that of 



