414 TRANSACTIONS OF SECTION C. 



flow as the Flysch that overlay the growing Alps. In all these cases, familiar to 

 us in Europe, the covering masses must have responded to the crumpling under 

 them, and, when reared to dangerous eminences, rapidly became a prey to 

 denudation and gravitational downsliding. They can Rrar(«ly be regarded as 

 protective, and their removal would leave the brittle masses below more liable 

 to fracture and to the ' calving ' process that forms klips."* 



Whether or no we postulate a yielding cover, the folding or overthrusting 

 thus becomes part of the phenomena on the surface of the globe. This fact is 

 glossed over in some of the admirable diagrams and models that have been 

 prepared for our instruction, where the Flysch, for instance, above the overfolds is 

 left without structural lines. Yet I venture to think, as I have already written,"^ 

 that in the Alps ' so much occurred within a single epoch, the Tortonian or 

 Upper INIiocene, and probably in a few thousand years, that some of the move- 

 ments must have been visible to the eye of man, had so discerning a creature 

 appeared upon the scene. Earthquake-shocks at the present day produce per- 

 ceptible imdulations of the ground, and may leave permanent traces in the form 

 of faults and dislocations. But it seems doubtful if a succession of small move- 

 ments such as man has been able to record represents anything like the building 

 of a mountain-chain when the resistance of the rocks has been overcome. . . . 

 Slow as the general movement may have been, the crumpling was not confined to 

 the hidden layers of the crust. It occurred in the rocks that formed the very 

 surface, and the final drop into the lowlands suggests the features of a land- 

 slide.' 



This conception was natural to the minds of our predecessors before a salu- 

 tary check was given to those who demanded frequent and world-wide ' revolu- 

 tions.' But nothing since that time has altered our impressions of the vast 

 forces latent in the earth, ready to perform work when unbalanced and set free. 

 The pressure that produces the solid flow foreseen by G. P. Scrope,'"' and 

 demonstrated in C. Lapworth's mylonites, is capable of rearing folds to dangerous 

 elevations at the surface. 



The danger lies in the form of the fold in relation to its base. Ampferer and 

 Hammer'" urge that the basal rocks are absorbed into the depths, since the 

 folded strata, when spread out again, would cover a far larger area than the 

 crystalline cores beneath them. The upper layers therefore tend all the more to 

 slide and fold on one another. Gravitation alone becomes under such circum- 

 stances a potent cause of surface-crumpling. 



G. P. Scrope ■" felt that the uprise of a chain was in itself sudden and 

 paroxysmal. We may go so far — or, shall we say, go back so far ? — as to realise 

 that large shifts may be made suddenly along thrust-planes when crushing takes 

 place and resistance has been overcome. We can feel greater confidence, how- 

 ever, when we consider the gravitational movements outwards from the line of 

 upfolding. These may be either one-sided or two-sided. Scrope " represents 

 the lower zone in a mountain-chain as flowing by pressure towards a line of 

 weakness, and the upper zone of rocks as flowing by gravitation away from this 

 line on both sides. He believed that, in addition to crumpling and the pro- 

 duction of recumbent folds, actual Assuring might occur. He explained in this 

 way the isolated blocks of the Dolomite Alps of Tyrol. ' Fracture chasms '-may 

 not have occurred between these particular blocks, though differential move- 



*' Something of this kind must have been pictured by C. L. Griesbach, when 

 he wrote ('Exotic Blocks of the Himalayas,' C.E., Congres gcol. internat., 1903, 

 p. 551) : ' Much of the older sedimentary rocks must have been brought to the 

 surface, not only as part of the sections, but also in crushed masses and detached 

 blocks torn off from situations in situ, a phenomenon common to all disturbed 

 areas. 'The outcrops of dislocations which have later undergone weathering and 

 denudation must, of course, have been shorn of all crushed and loose frag- 

 mentary masses.' 



"= The Growtli of Euroff. (1914), p. 173. 



" Covxiderations on Volranox (1825), pp. 202 and 234. 



" Op. fit. (ref. 38), p. 708. 



** CoyhtuJe rat ions nil Volrnnns (1825), pp. 202 and 234. 



" Ihich, pp. 202 and 204. 



