286 W. BOWIE THEORY OF ISOSTASY 



to make some change, and this is far better than many of the older geo- 

 logical theories regarding uplift and subsidence which had no apparent 

 cause for the change. 



Aside from the geodetic evidence against regional horizontal forces 

 acting to throw up mountains, we must consider that the material in 

 mountain areas which show horizontal movements is usually quite thin. 

 A layer of material a mile in thickness is about the maximum that we 

 might expect to find. With material of such thinness, being merely a 

 veneer on the earth's crust, we should not expect sufficient strength to 

 justify the idea that the movement has been due to forces acting through 

 hundreds of thousands of square miles of area outside of the mountain 

 region. Such thin material would certainly buckle locally rather than 

 transmit stresses for great distances. This material is frequently warped 

 in the areas outside of the mountains, and therefore any tendency of a 

 regional horizontal force to act through this thin disk would undoubtedly 

 cause it to buckle locally and not to transmit the force necessary to cause 

 distant uplift. In addition, the force would have to overcome the tre- 

 mendous resistance to shear between this thin veneer and the material 

 of the earth's crust below, it. 



An analysis of the mechanics of the problem shows so many reasons 

 why regional forces can not be the cause of distortion that we must aban- 

 don the whole theory of regional horizontal thrusts and accept one that 

 is in closer harmony with the data available and that will not conflict 

 with the usual processes of mechanics. 



Reid 9 some years ago, and recently Burrard, 10 concluded that the theory 

 that mountains are caused by regional horizontal forces is not in accord 

 with the theory of isostasy. 



There is evidence that there are many places which have been uplifted 

 or depressed by very small amounts in comparison with the movements 

 that have occurred in mountain areas. In fact, there is scarcely any part 

 of the earth's surface that has not been elevated or depressed, relative to 

 the position held at some former geological period. These movements, 

 while of very much less importance than the great movements caused in 

 areas of heavy sedimentation and mountain uplift, are nevertheless im- 

 portant from a geophysical and geological standpoint and should be con- 

 sidered. But the principal thing is to arrive at some definite conclusion 

 in regard to the cause of the larger vertical movements. 



9 H. F. Reid : Isostasy and mountain ranges. Proceedings of American Philosophical 

 Society, 1911, pp. 444-451. 



10 Sidney Burrard : On the origin of mountains. Geographical Journal. September, 

 19L>1. 



