464 SCIENCE PROGRESS. 



conditions appear so well marked out on the northern 

 hemisphere, they fail hitherto to explain the character 

 of the southern. 



Discussing the question of faulting in its relation to fold- 

 ing, he points out that faults are most frequent in the great 

 plateaux, and in feebly undulated beds ; and considers that 

 these arise from the parts insufficiently supported from be- 

 low yielding under the influence of gravitation ; and that 

 the relative play of the component parts, and the deeper 

 sinking of some of them, may be referred to the influence 

 of centripetal action arising from the secular cooling of the 

 planet. 



Again, in zones where folding is active, faults, if pro- 

 duced, are a resultant of that action, and it is along their 

 lines of fracture that pressure continuing induces sliding ; 

 but only from below upwards, thus causing the superim- 

 position of the older beds on the younger. These faults 

 are therefore inverted, and in so far as they affect moun- 

 tains, only occur in the exterior or sub-Alpine zones. On 

 this hypothesis, therefore, the fold is the element, the prin- 

 cipal phenomenon, and the fault a detail arising from its 

 action. In the central parts of the chains, however, 

 where cohesion is too strong to allow of faulting along 

 any other line than that of stratification, gliding and 

 slipping has only occurred along the lines of least resist- 

 ance ; the softer beds being crushed out, the harder ones 

 would retain an apparent conformity in their stratifica- 

 tion, but the general effect of the combined movements 

 would be the thinning out of the whole series. It will 

 thus be seen that in so far as it regards the horizontal 

 displacement of an overlying fold, its being drawn out 

 under the influence of continuous pressure, and the crush- 

 ing and thinning out of the beds of the inverted arch, 

 when the latter is completely superposed on the strata 

 forming the trough, in all these points Bertrand is in com- 

 plete accord with Heim. 



The various theoretical deductions, however, which dur- 

 ing the last fifteen years have so greatly influenced geo- 

 logical thought, have not passed unchallenged, and even 



