FOLDS AND FAULTING. 467 



independent of the fold movement. Though there may be 

 isolated cases of absolute correspondence of the overthrust 

 surface and the axis plane, yet the examples where the 

 other conditions prevail are so much more numerous that 

 they should be considered as exceptional and not in essen- 

 tial relationship. 



(5) As a rule the strata both above and below the thrust 

 plane are normal — that is, the younger overlie the older. 

 When the thrust plane has a very feeble slope, and stratifica- 

 tion is simple, the result is the repetition of the same series 

 of strata ; should, however, these be much dislocated and 

 compressed into many folds, they are, apparently, more com- 

 plicated, and the series may be repeated several times 

 where folds are very inclined, lying sometimes on the thrust 

 plane in a normal, and sometimes in an inverted, condition, 

 this latter being invariable when the lower limb of a lying- 

 trough has been overthrust by the upper one. In cases 

 where the thrust distance has not been an extended one, or 

 the direction of thrust has been at a highly-inclined angle, 

 the tectonic relations of the troughs and saddles are not 

 completely obscured ; but where the overthrust has extended 

 several miles, and exceeds the breadth of the folds, and 

 still more so, should the thrust plane be horizontal, then the 

 original tectonic relationship will be scarcely recognisable, 

 and the nature of the strata below and above the overthrust 

 may be completely different in character. 



(6) The rocks bordering the overthrust plane have in 

 almost every case undergone mechanical and chemical 

 alterations of an intense character. This ordinarily consists 

 in the formation of coarse breccias or fine sands through 



o 



disintegration of the rocks. Owing to chemical solution and 

 recrystallisation, the breccia rock may differ markedly from 

 that whence it was derived, polished planes and cleavage 

 being produced, giving rise to structures such as fault rock, 

 mylonite, etc., which are, all of them, more or less parallel 

 to the thrust plane. 



These structures may also, in many cases, be easily re- 

 cognised as having been produced through the grinding of 

 the constituents along a common friction plane, as when two 



