MECHANICS OF FORMA TION OF ARCUA TE MOUNTAINS 1 85 



— rather than close responses — should therefore be expected to 

 connect the growth of mountains and their attendant seismic dis- 

 turbances with volcanic manifestations. The variations in the 

 competence of the rising arch particularly after underturning has 

 begun, considered in connection with the incidents of failure and 

 their attendant consequences (hereafter to be discussed), are such 

 as to make probable long-period variations particularly, both in 

 seismic disturbance and in volcanic extravasation. 



B ackf aiding of anticlines. — As yet comparatively little attention 

 has been given by geologists to the effect of the local occurrence of 

 weak or strong facies of a formation or series upon the character 

 of the folds produced in them.^ The subject is too complex and 

 too little known to be discussed at length, but there are yet some 



Fig. 27. — Backfold induced in a leaden strip above a local zone of weakness (after 

 Daubree's experiment) . 



very significant indications of its importance in fixing the location 

 of the rare backfolds which have sometimes been described. In 

 this connection a simple experiment by Daubree is illuminating.^ 

 The strip of lead, which in so many of his tests was compressed from 

 the end by a piston, was in one case locally weakened by thinning at 

 some distance from the piston head, and the amplitude of upward 

 deflection was limited by a horizontal beam above. The flexible 

 strip was under these conditions deformed into a true backfold 

 (Fig. 27), although it failed to simulate the Glarus double-fold as 

 had been intended. 



Nature has furnished an even better illustration in the Weis- 

 senstein of the Chain Jura, which the series of parallel profiles by 



' Paulcke's experiments fail to afford altogether satisfactory results for the reason 

 that his competent member is insufficiently loaded and does not fold. 

 ^ Op. cit., p. 296, Fig. 85. 



