Shaler.] 14 [June 6, 



place, and the isogeotliermals, 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 tlif; land surfaces, and seldom or never originate on the ocean 

 floors. On the land ai'eas we would expect to find them originating 

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

 displacement of the beds constituting the crust, from their normal 

 position, for at such jioints 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 dejiosition 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 cr-nst 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 sedi- 

 mentary materials had, at the same time, prepared that portion of the 

 crust beneath the ocean floor for subsidence, then the moment this 

 latter action is effected, it is likely to bring about fractures along 

 shore lines, attended by the escape of gaseous and igneous materials. 

 This dislocation of the crust would be attended by the pushing to- 

 gether of the superficial poi-tlons from either side, and the resulting 

 elevations might be complicated by the intrusion of a greater or le.-s 

 amount of igneous matter. 



This view of the origin of mountain chains seems to be reconcil- 

 able with some of the most prominent features which are to be found in 

 their structure and distribution. Their usual, if not Invariable, origin 

 alono- shore lines, the suddenness of their formation, the variable 

 amount of igneous action exhibited in their masses, are explicable on 

 this hypothesis. On the other hand. It is not to be denied that some 

 considerable objections, can be urged against it. In the first place, 



* See these Proceedings, Vol. x. page 237. 



