ISOCLINAL FOLDS 



797 



preted as all belonging to one limb of a fold, and therefore repre- 

 senting a continuous series, or as representing one or more isoclinal 

 folds. According to the first interpretation, the sediments have 

 great thickness, and there is a recurrence of similar beds, while, 

 according to the second interpretation, the series is much thinner 

 and the recurrence of beds is only apparent, there being an actual 

 repetition of the same beds. The problem is often a difficult one to 

 solve, and depends upon the identification of the similar beds as 

 parts of the same bed. A knowledge of the degree of folding char- 

 acteristic of the region in question and a knowledge of the charac- 

 ters and thicknesses of the formations involved in other and undis- 



123 4 4 3-'l 123443 



1/ ' / 





Fig. 179. Isoclinal strata, showing repetition of strata (0) and two methods 

 of reconstructing them (&, c). 



turbed regions will often serve to settle the question. If more than 

 two kinds of beds are involved, the order of repetition will often 

 give a clue to the original condition. Thus in a closely folded dis- 

 trict, the strata within the same fold will be repeated in inverse 

 order, as shown in Fig. 179, a. 



This generally is conclusive evidence of repetition by folding. If 

 this point is settled, the next question is : Which bed is the upper 

 and which is the lower of the series? If the character of the folds 

 can be determined by inspection, the proper relation will at once 

 appear, for if, when beds 4 and 4 come in juxtaposition, they pre- 

 sent the upper ends of the fold, i. c, they are parts of an anticline, 

 bed 4 is the oldest and originally the lowest of the series (Fig. 

 179, b). 



If, however, the two limbs of the folded bed 4 join below the 



