458 SCIENCE PROGRESS. 



existing arch, which has not succeeded in developing itself, 

 because it was overlaid by an enormous mass of Flysch 

 formation, and it has therefore been disjointed, and trans- 

 formed into an anticlinal overthrust ". 



Lateral over thrusts are more limited, and are due to the 

 sinking of one side of an arch, the downward movement 

 being compensated by the contortion of the marly beds 

 forming the core of the overlying fold. In Isoclinal over- 

 thrusts one of the parts is so covered by the other that the 

 beds in both appear to be nearly parallel, and where harder 

 beds overlie softer ones, experiment has shown the possi- 

 bility of a synclinal break ; one of the composing anticlinal 

 limbs being so broken as to produce an underthrust into 

 the softer beds below (see plate ix. of above work). The 

 final result of these studies has therefore again been to 

 confirm, in his opinion, in a great measure Heim's views 

 that overthrust faulting as the result of folding is a pre- 

 dominant rule in mountain structure. 



On the basis, therefore, of a number of ascertained 

 facts, in 1886 a very considerable amount of materials had 

 been collected, and certain broad generalisations announced. 

 It had been shown that mountain structure is not merely 

 local, but the result of a movement affecting a broad belt 

 around the globe, a considerable surface being involved, 

 which has a distinct flow structure in a certain direc- 

 tion, giving rise to faults and sinking areas in the rear. 

 That the whole of this broadly separated region has 

 been folded and contorted, every part of the area being 

 more or less affected by the movement, though the central 

 portions have undergone the greatest flexuring. That 

 normal faulting, as such, has played but little part in moun- 

 tain structure ; the fan structures, and overthrust folding or 

 faulting being in the main resultants of lateral compression. 

 That the formations of the valleys and outlines of the 

 peaks have been entirely the work of erosion and denuda- 

 tion acting through prolonged periods, whilst the variations 

 in the character of strata have oriven rise to all the com- 

 plexities of detail which for so long a time rendered the 

 Alpine regions a geological puzzle. It has also been ad- 



