FOLDS AND FAULTING. 473 



are more highly complicated, owing to the original folding 

 having been much masked by subsequent faulting, yet in 

 the main the same general lessons are to be learnt. Roth- 

 pletz, in dealing with the district of Provence, lays special 

 stress upon the fact that here also there is no absolute 

 verification of the " squeezing out of the middle limb ' : 

 theory. He considers that without its aid all the profiles may 

 be explained, and that, in view of this uncertainty, it would be 

 advisable to reject the term pli-faille, substituting for it one 

 without theoretical signification, such as pli de recouvrement. 



It cannot be doubted, however, that in this part of 

 France folding and overfolding of the strata have been 

 antecedent, and that subsequently and locally the folded 

 masses have been thrust over each other along fracture 

 surfaces inclined at a low angle ; also most of the dis- 

 locations and depressions have taken place along fissures, 

 which have affected both fold and overthrust. 



Finally, he remarks that long since in America Professor 

 H. D. Rogers had recognised the fact that overthrust could 

 take place along a fault plane developed parallel to the axis 

 plane of an anticlinal fold. 



A review of the whole subject of mountain building 

 reveals to us the existence of a number of conflicting 

 opinions, attempting to explain by various methods a series 

 of well-ascertained facts. 



Even the main theoretical basis is by no means definitely 

 settled, or universally accepted. Thus, while the greater 

 number of observers are agreed to consider the slow cooling 

 of the earth's surface, and its consequent contraction, as the 

 chief cause of its present broad features, others, notably Mr. 

 Mellard Reade, hold the view that expansion has played 

 the most prominent part ; that in regions of depression, 

 where accumulation of sediments is continuing, there is a 

 gradual rise of the isogeotherms as we descend ; that from 

 the expansion produced by this rise upper curvature of 

 the underlying rocks will eventually be produced ; and also 

 that from the effect of this expansion on the overlying beds 

 vertical movement will result, giving rise to the formation 

 of mountain chains. 



