278 w. bowie theory of isostasy 



Depth of Compensation 



In our isostatic investigations we have assumed uniform distribution 

 of isostatic compensation with respect to depth. This was done for two 

 purposes : In the first place, to make it possible to carry on the great 

 amount of work involved in computing the topographic effect on the 

 plumb line and on the pendulum, and in the second place because uniform 

 distribution is a mean of all the possible methods of distribution. We 

 do not, of course, feel that the compensation is exactly uniformly dis- 

 tributed, but it is probable that, no matter what the distribution may be, 

 the effect on the geodetic data would be about the same as uniform dis- 

 tribution. At least the uniform distribution has had a remarkable suc- 

 cess in eliminating deflections of the vertical and in reducing gravity 

 anomalies without leaving any apparent relation between the topography 

 and the residuals. 



From the investigations involving the deflections of the vertical and 

 isostasy and those dealing with the values of gravity and isostasy, depths 

 of compensation have been derived. The depth of compensation is a term 

 used to indicate that there is a surface concentric with sealevel below 

 which there is no isostatic compensation. It is not probable that the 

 depth of compensation is a definite surface, nor is it probable that the 

 compensation extends always to the same surface or zone. It is certain 

 that we do have somewhere below sealevel such changes in the conditions 

 of temperature and pressure that the material of the crust is transformed 

 from a resisting to a yielding mass. 



We have a horizontal movement of the material somewhere below the 

 surface of the earth's crust which tends to maintain the equilibrium be- 

 tween blocks of the isostatic shell. It is well known that tens of thou- 

 sands of feet of material have been eroded from mountain areas and de- 

 posited at low altitudes. The gravity results and the deflections of the 

 vertical show that areas of erosion and areas of deposition are approxi- 

 mately in equilibrium, and therefore we must have had transference of 

 material from the column under the sedimentary area back to the column 

 under the area of erosion. The question is, Where does this flow take 

 place ? 



The material of the earth's crust offers great resistance to horizontal 

 movements within it, although it yields vertically rather easily to loads 

 that are laid down on its surface. If the material of the crust itself were 

 plastic to horizontal forces, we might expect mountain masses to overflow 

 surrounding plains and the continents to flatten out into the ocean bot- 

 toms. This does not happen. It is reasonably certain that the material 



