STRUCTURE OF LAND AREAS 



81 



Vast layers of rocks have been worn away by the forces 

 which are continually wearing away and removing the 

 rocks at the surface of the earth, and thus rocks which 

 were once at great depths below the surface have been ex- 

 posed. Even granite rocks which were originally formed 

 at a depth of thousands of feet below the surface now ap- 

 pear at the surface and 

 are being quarried in 

 many places. 



The folding and warp- 

 ing of the rock layers 

 has brought some of the 

 stratified beds which 

 were originally horizon- 

 tal into an almost verti- 

 cal position so that we 

 now find at the surface 

 the worn-off edges of 

 these beds. The dif- 

 ferent kinds of rocks 

 and the different posi- 

 tions in which the rock layers are presented to the forces 

 which are active in wearing them away cause great variety 

 in the forms of the surface features. 



It is not necessary to consider all the causes which may 

 have disturbed the position of the rock layers, but the 

 most important of them deserves attention. It has already 

 been found that, although the exterior of the earth is cool, 

 the interior is hot. Now it is known that almost all sub- 

 stances contract when cooled. If the interior of the earth 

 is cooling, and there is every reason to believe that it is, 

 then it must be contracting. As the crust is already cool 

 it has ceased to contract and thus the interior shrinks 

 away from it and it must fold up in order still to rest 



FOLDED ROCKS. 



Stratified rocks which have been folded 

 since they were formed. 



