2IO FRANCIS PARKER SHEPARD 



MOUNTAIN GROWTH WITHOUT LOSS OP ISOSTATIC ADJUSTMENT 



In regard to the possibility of change from geos3niclines to 

 mountain ranges without altering the isostatic equilibrium, Bowie 

 offers an explanation which disregards lateral shortening/ The 

 gist of this is that while the geosynchnes are subsiding, the isogeo- 

 therms are depressed beneath them. When subsidence ceases 

 these isogeotherms rise to their former position. The resulting heat- 

 ing of the mass is supposed to be the cause of the mountain upKft 

 by heat expansion. It seems rather doubtful that the slow sink- 

 ing of a geosyncline would cause a depression of the isogeotherms, 

 but granted that this would actually occur, it must be remembered 

 that subsidence of such geosynclines as the Appalachian was often 

 interrupted and for long epochs the geosyncline was not covered by 

 the sea. If we accept Bowie's idea, we have to account for the lack 

 of mountain building during each of these intervals. The idea there- 

 fore does not agree with accurate geological observations. 



Considering mountain building as the product of compressive 

 forces. Wood has made the important suggestion that the arching 

 and folding of mountain ranges relieves the burden on the earth's 

 interior^ and therefore allows the range to stand up without applica- 

 tion of extra weight. This explanation does not account for the 

 geodetic observations which show no extra gravity pull for the 

 mountain ranges since the mass of material beneath the range is 

 not changed just because it does not bear down as much on the 

 interior. It is the mass that produces gravity pull. While this 

 may be the cause which initiates the mountain building, something 

 else is necessary to account for the lack of anomalies. 



If we examine the history of the geosyncline and range develop- 

 ment, we can find a possible explanation. While the geosyncline 

 is sinking, the material in it and beneath it is getting continually 

 more burdened and farther from the surface. If rock material may 

 take on more compact forms as it becomes more compressed,^ 

 these layers under the geosyncline would be changing into denser 



' W. Bowie, Geographical Review, October, 1922, p. 623. 



" W. O. Wood, Bull. Geol. Soc. America, Vol. XXXIII (1922), p. 312. 



3T. C. Chamberlin, Jour, of Geol., Vol. XXIX (192 1), p. 679. 



