STRUCTURAL GEOLOGY 405 



Much the larger portion of the earth's surface is occupied by 

 beds that depart but little from their original horizontal attitude, 

 but in mountainous regions the beds have usually suffered bending, 

 folding, crumpling, and crushing, in various degrees, in the course 

 of the deformations that gave rise to the mountains. Distortion 



Fig. 322. This diagram might represent either isoclinal or monoclinal 

 structure. In the former case, the strata might have the structure 

 shown in any one of the figures 323 to 325, so far as dip and strike show. 

 (Dana.) 



is on the whole most intense and characteristic in the most ancient 

 rocks. Distortion is assigned chiefly to lateral thrust arising from 

 the shrinkage of the earth, as explained in Chapter IX. 



Complicated structures may be very difficult of interpretation. 

 Thus overturned folds reverse the order of the strata in the under 

 limb of the fold (Fig. 311). After such folds have been greatly 



Fig. 323. Fig. 324. Fig. 325. 



Fig. 323. A possible interpretation of Fig. 322. (Dana.) 

 Fig. 324. A possible interpretation of Fig. 322. (Dana.) 

 Fig. 325. A possible interpretation of Fig. 322. (Dana.) 



eroded, so that their outer form is lost and their relations have be- 

 come obscure, the reversed beds are likely to be interpreted 

 as though they lay in natural order. In such a case as that 

 represented in Fig. 322, a complex structure may be interpreted 

 as a simple one. Thus these strata may have the structure shown 

 in Fig. 323, 324, or 325, so far as dip and strike show. 



