INTERCHANGE OF SEA AND LAND' 249 



interior shrinks, the crust must fold up in order still to rest 

 upon the shrinking interior. The wrinkling of the skin of 

 the baked apple as the interior of the apple cooled gives 

 a faint notion of what has been happening to the crust of 

 the earth through the ages. The cooling of the earth is so 

 slow that the folding usually disturbs the surface but little 



FOLDED STRATA 



at a time. In recent hundreds of thousands of years, there- 

 fore, geological changes have usually taken place very 

 gradually. These slow changes are still continuing, and 

 the surface of the earth is being constantly modified. 



Interchange of Sea and Land. In many places at 

 considerable distances from the ocean, sea shells have been 

 found in the crust of the earth. Tree trunks are sometimes 

 found at considerable depths in the sea, standing with 



