246 



THE NATURE BOOK 



iiir. >jK.-A,\ilh PiN:N/\i„i,r,:5 >.iF THE MOUNTAINS OF CORSICA 



any considerable de])th, but show alter- 

 nations of strata with varying degrees 

 of resistance. It is a common sa3'ing 

 that the strength of a chain is the strength 

 of its weakest link. The same holds 

 good with rocks of this class. A layer 

 of soft yielding shale interbedded with 

 more durable limestone or sandstone 

 may cause the whol<3 to decay as rapidly 

 as the shale itself. 



As a rule, the strata arc laid down 

 horizontally, but in the process of moun- 

 t a i n building 

 they may 

 become tilted 

 into an inclined 

 position or bent 

 into folds with 

 crests and 

 troughs like the 

 waves of the 

 sea. 



It has been 

 proved, experi- 

 mentally, that 

 many i' o c k s 

 whicli would 

 break witli a 

 sudden blow 

 will bend if the 

 ])ressure be ap- 

 1)1 ied very 

 gradually and 

 continued for 



a long time. Hence we 

 do not postulate that a 

 sudden or violent force 

 has crumpled up the 

 limestone bands shown 

 in the photograj)!! on 

 ]mge 245, but rather a 

 gradual pressure lasting 

 through immeasurable 

 time. 



If the distorting forces 

 act more ra]iidly, we get 

 fracture instead of bend- 

 ing, and great Ijlocks of 

 rock, sometimes miles in 

 extent, may slide over 

 each other along the line 

 of breakage. Displace- 

 ments of this nature are 

 called " faults." 



The grinding of great 

 rock masses over each 

 other also results in other effects than 

 mere displacement. \'ibrations are set 

 up in the adjacent rocks which produce 

 the phenomena of earthquakes. As many 

 thousands of earthquakes occur in the 

 Earth's crust every 3'ear, and each one 

 of these results from the fracture and 

 slij)ping of strata, we see that the Earth is 

 still subject to great stresses, and is 

 constantly accommodating itself to them. 

 Now let us consider the external forces 

 whose office it is to degrade the land. 



"MASSES OF GRANITE MAY BE SHATTERED INTO FRAGMENTS AND 

 LIE ABOUT. A PICTURE OF RUINOUS DECAY." 



