720 recumbent polds produced as a result of plow. [nov. 1906, 



Discussion. 



The President remarked that it was a source of gratification to 

 the geologists of this country to learn that the types of mountain- 

 structure which were first worked out in detail in the North- West 

 of Scotland had been found so widely distributed over the globe, 

 and that the conclusions regarding them had been so fully accepted 

 by geologists in other lands. Whether or not the extremes to which 

 these conclusions had been pushed by some of the more enthu- 

 siastic spirits who had taken them up would ultimately be justified, 

 time must show. In such papers as the present, there always seemed 

 to the speaker to be two distinct, but closely- related aspects of interest. 

 On the one hand, the questions of physical structure possess an 

 absorbing fascination : every year the answers to them are be- 

 coming clearer, and our conceptions of the manner in which the 

 crust of the earth has been shaped into its present condition are 

 growing more definite. On the other hand, the solution of these 

 physical problems brings ever into more vivid prominence the 

 stupendous extent to which the surface of our globe has been 

 affected by denudation, and the prodigious length of time during 

 which this slow process of sculpture must have been at work. 

 Even if such gigantic displacements as some observers maintain to 

 have occurred, were eventually found to be somewhat exaggerated, 

 there can be no doubt that the types of mountain-structure which 

 have been proved to exist, while they furnish fresh demonstration 

 of the potency of denudation, supply valuable evidence as to the 

 high antiquity of our globe. 



Mr. A. P. Young suggested, in relation to the Author's theory 

 requiring the presence of massive obstacles, that it might be 

 profitable to discuss the hypothesis framed by geodesists to explain 

 the results of pendulum-observations, which require a defect of 

 mass under high mountains. 



M. M. Allorge pointed out the close correspondence between the 

 structure of the northern side of the Western Alps and the results of 

 the Author's suggestive experiments. In the central part of the Alps 

 there are two series of elliptical granitic masses, the major axis of 

 which indicates the trend of pre-Permian (Hercynian) folds : the .Belle- 

 donne zone, and the Pelvoux-Mont-Blanc zone. When the Tertiary 

 overfoldings took place, in consequence of pressures coming chiefly 

 from the south-east, these pre-existing Hercynian horsts were able 

 to play a part similar to that played by the two obstacles of the 

 Author's experiment. They have very likely strengthened the 

 tendency of the folds to form secondary undulations, and to leap one 

 over the other. He considered that the Author's results afforded an 

 experimental verification of the law established by Prof. Lugeon, 

 namely, that the internal folds first produced are the shortest. The 

 most external are the longest, and overlap those which precede them. 

 The difficulties sometimes encountered in attempting to correlate the 

 front of the sheets with their roots are due to the fact that their 

 present separation is not only caused by recent erosion, but some- 



