336 S. BU.RRARD FOLDING OF MOUNTAIN RANGES 



the crust by compensating at depth the effects of surface erosion and 

 deposition. 



But a different form of conclusion has been drawn from the theory of 

 isostasy, one which has not been based on geodetic observations. In com- 

 pany with other geodesists, I have myself in the past fortified my convic- 

 tions by means of this very conclusion of which I am now so doubtful. 

 It has been argued that the theory of isostasy excludes all hypotheses 

 which give to mountains additional mass, and that therefore mountains 

 can not have originated from horizontal folding, because horizontal com- 

 pression and folding imply additional mass pushed in from the sides. 



Of recent years geodetic observations have shown that however rock- 

 mass may be moved about over the surface of the earth, the changes in 

 its surface position become compensated at depth. If mass is eroded 

 from a mountain peak, the loss from erosion becomes locally compensated, 

 and if mass is deposited in alluvial beds, the additional load becomes 

 compensated; that is to say, when rock is moved horizontally over the 

 earth's surface from one place to another, the horizontal transfers of 

 matter are compensated by changes of density in the crust. 11 From 

 gravity observations taken at stations near the mouths of the Mississippi, 

 the Ganges, the Irawadi, and the Po, it appears that the deltaic deposits 

 of great rivers are compensated. 12 In all probability the deltas of the 

 Nile and the Tigris will be found to be compensated, when the necessary 

 pendulum observations have been made. These deltas are additional 

 amounts of material brought in horizontally from the side, just as the 

 folds of strata are. If these deltaic loads, brought in from one side, be- 

 come automatically compensated, why should not loads which have been 

 accumulated locally by folding be also compensated ? 



I agree with Bowie's contention that mountains can not have origi- 

 nated by folding, but I think that the hypothesis of the folding of strata 

 will have to be challenged by its opponents on other grounds than that 

 of isostasy. 



11 Hayford and Bowie : Effect of topography and isostatic compensation upon the in- 

 tensity of gravity, 1912, p. 112. 



12 William Bowie : Investigations of gravity and isostasy, 1917, p. 84. 



