GEOLOGIC STRUCTURE 



51 



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modified by the presence of tilted fault blocks whose longer axes are par- 

 allel to the trend of the anticlines. Each of the mountain ranges, or 

 sierras, which have been de- 

 scribed above is a long, nar 1 

 row, sharply folded anti- 

 cline. The length of these 

 folds is in each case ap- 

 proximately equivalent to 



the length of the sierra and 5 



is, therefore, to be meas- g 



ured in scores of miles. £ 



The width of each fold is I 



considerably greater than |' 



the width of the mountain g 



range for which it is re- | 



sponsible and varies be- o 



tween 5 and 15 miles. « 



The strata forming the © 



limbs of the anticline are * 



tilted at angles of 25 de- 3 



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grecs to 75 degrees, or even * 



more; their average slope is » 



about 40 degrees. Strictly 3 



speaking, each anticline is «■ 



doubly plunging, for each g 



dies out at either end, but f 



the length of the folds is so 3, 



"Teat that their crests are » 



<^ « 



practically horizontal for ^ 



great distances. Their axial « 



O Co 



planes are seldom vertical, §■ 



but more or less toward the £ 



east, suggesting that the s* 



great thrust which crumpled ~~ 



the region came from the | 



west. Each of the great » 

 anticlines, with the excep- 

 tion of tbe Cbaragua anti- 

 cline, is sheared along a 

 major fault - plane which 

 trends parallel to the anti- 



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