18 W. Boivie — Relation of Isostasy to 



"VYe have evidence that much material of the earth has 

 been uplifted without distortion. The great plateaus of 

 the west have their strata practically horizontal and in 

 some cases they extend for miles. These plateaus were 

 originally at or below sea level and it is improbable that 

 the material of the column of the isostatic shell under 

 them was less dense than normal. Therefore, when they 

 have been elevated, and the geodetic data prove that they 

 are' now in approximate equilibrium, we must conclude 

 that no additional material has been added to the columns 

 under the plateaus. 



Barrell recognized the necessity for having a decrease 

 of density under the Colorado plateau. In the ^^ Strength 

 of the Earth's Crust" he said: 



''It is known that a region like the Colorado plateau, which 

 now stands markedly high, tended to lie near sea-level from the 

 beginning of the Paleozoic to the end of the Mesozoic. Pre- 

 sumably a decrease of density within the zone of isostatic com- 

 pensation has taken place here during the Cenozoic and the 

 uplift has accompanied or followed the internal change. ' ' 



Great changes in elevation not due to isostatic adjustment. 



It must be clearly borne in mind that the theory of 

 isostasy cannot explain great changes in elevation. There 

 will, necessarily, be some changes in elevation which can 

 be attributed to the theory of isostasy. These changes 

 result from lighter material than normal being deposited 

 and sinking to take the place of material of normal 

 density. If no other action than isostatic adjustment 

 acted in the column under the sediments we should expect 

 the surface of the sedimentary material to increase grad- 

 ually in elevation. If, for instance, we had 30,000 feet 

 of material deposited we should expect at least three 

 thousand feet to project above the original position of 

 the base of the sediments. As a matter of fact, it is 

 probable that the elevation of the surface would be even 

 higher than that, because the difference in density 

 between the sedimentary material and material displaced 

 by the sediments is undoubtedly greater than ten per cent. 



As erosion continues in a mountain area the surface 

 should become gradually lower as the result of the isos- 

 tatic adjustment. The reason for this is that material 

 is brought into the column under the mountains heavier 



