474 SCIENCE PROGRESS. 



Putting aside, however, any further discussion of the 

 views held by many most competent observers, we would, 

 in conclusion, place in as sharp a contrast as possible the 

 two broad principles which we have endeavoured to review. 



The first, originally propounded by Heim, may be 

 summarised : mountain structure is the direct result of 

 folding. This has taken place far beneath the earth's sur- 

 face, the rocks affected being, under the influence of enor- 

 mous pressure, in a condition of latent plasticity, consequently 

 faulting in the mass becomes impossible. Overthrust is the 

 resultant of extreme overfolding, the middle limb of a fold 

 being drawn and squeezed out between the arch and trough. 

 Subsequently, erosion, having removed the great thicknesses 

 of superjacent rock-masses, has revealed the folding, and at 

 the same time caused the varied features of the mountain 

 chains. 



The second, of which Rothpletz is the exponent, formu- 

 lates : folding is the first step in mountain building as a 

 result of lateral pressure, but, as in Daubree's experiment 

 (fig. 3), on upheaval, part of the mass ceases to be affected 

 by such pressure, great fault planes are produced, along 

 which parts of the already folded beds are thrust over the 

 younger ones. Overthrusts, therefore, are really faults of 

 a special type ; and the more normal form succeeding them 

 determines the formation of valleys and peaks, lakes and 

 precipices. The first connects overthrust with folding, the 

 second with faulting. The one demands enormously thick 

 overlying deposits and erosion acting through long periods ; 

 the other, no special conditions, and attributes the present 

 features of the earth's surface mainly to faulting. 



In the Livret Guide, issued to the members of the 

 late International Geological Congress in Zurich, the Swiss 

 geologists have endeavoured to explain the inner structure 

 of the Alps, and no one can fail to be struck with the im- 

 portance which the character of the deposits has had on 

 the nature of the rock-foldings. In the section illustrating 

 Schardt's Alpine excursion the failles de recouvremenl are 

 seen both to the north and south of the Pays d'Enhaut 

 to have formed between the Eocene shales and the 



