516 Notices of Memoirs — N. S. Shaler 



would expect the fracture to take place on the surface, rather than 

 below. There is one thing which could operate to prevent the cer- 

 tain contortion of the superficial portions of the crust, and that is the 

 horizontal j^osition of its beds ; as ordinarily constituted, the resist- 

 ance which the upper few miles of the crust could oppose to the 

 action of any force tending to throw it 'into folds, is very great. 

 When the contortion has once begun, and this resistance fairly over- 

 come, all further changes would meet with comparatively little 

 resistance. 



We have spoken only of those cases where the original surface had 

 continued to exist from the beginning, while the isogeothermals 

 beneath them had gradually sunk deeper and deeper towards the 

 centre of the earth. This being a very unlikely condition, it remains 

 to be seen what would be the effect where the actions of denudation, 

 or deposition, are going on. It is evident that whenever the rate of 

 denudation was such that the removal of the crust took place with 

 the same rapidity as the recession of the isogeothermals from the 

 surface, there could be no lateral strain produced by the loss of heat. 

 Where, on the other hand, rapid deposition of materials was taking 

 place, and the isogeothermals, on that account, were rising towards 

 the surface, there would also be no such strain on the upper part of 

 the crust. It thus appears that the conditions of tension competent 

 to produce mountain chains, would only be found strongly developed 

 in regions where the rate of denudation was less than the rate of re- 

 cession of the isogeothermal lines, or where the rate of deposition 

 was not sufficiently rapid to prevent the recession of the lines of 

 equal heat. 



Accepting this hypothesis of the origin of mountain chains, it is at 

 once seen that they should have their region of greatest development 

 on the land surfaces, and seldom or never originate on the ocean 

 floors. On the land areas we would expect to find them originating 

 at those points where there were some forces operating to favour the 

 displacement of the beds constituting the crust, from their normal 

 position, for at such points the contracting force would most easily 

 produce corrugations. The author has elsewhere given a brief notice 

 of a view of the origin of continents, from the tendency of all 

 regions where deposition is going on, viz., sea bottoms, to subside.^ 

 This view, if correct, will warrant us in believing that shore lines 

 are points where fracture and dislocation of the crust are likely to 

 occur. The distribution of volcanic vents of the present day, and 

 the instructive fact that volcanic outlets of former geological periods 

 ceased to be active when left inland, in the progress of geological 

 changes, would of themselves indicate a peculiar liability to rupture 

 of the superficial portions of the crust along shore lines. Let us 

 suppose that the recession of the isogeothermal lines had placed the 

 superficial portion of the crust in a state of tension, which could 

 only be relieved by the formation of mountain elevations, and that 

 the laying down of sedimentary materials had, at the same time, 

 prepared that portion of the crust beneath the ocean floor for subsi- 

 ^ See these Proceedings, Vol. x. p. 237. 



