YIELDING OF THE EARTH 's CRUST BOWIE. 243 



next stage in its geological history. This downwarping undoubt- 

 edly must be due to an increase in the density of the material of the 

 block of the earth's crust under the area, which results in a shorten- 

 ing of the block and, consequently, in a lowering of the surface. We 

 are forced to this conculsion because otherwise there would be a 

 change in the mass of the blocks upsetting the isostatic equilibrium 

 at its base. The results of the geodetic investigation indicate very 

 clearly that the blocks are in approximate equilibrium and that the 

 deviation from the perfect state of equilibrium is very small. 



The contraction of the column and the increase in the density 

 of its material probably is due to a process which is just the reverse 

 of the one which causes the expansion of a column resulting in the 

 uplift of mountains. 



While an elevated area is being eroded, the column under it is 

 continually rising, because of the material which enters at the lower 

 end of the column or block. This material is moved into the block 

 by what is called the isostatic adjustment. Since the surface of a 

 block that is undergoing erosion is lowered by only about one-tenth 

 of the thickness of the matter which is eroded, there must be tens of 

 thousands of feet of material carried away before the surface of the 

 block is base-leveled. 



Let it be assumed that 30,000 feet of matter have been eroded from 

 a certain elevated region. Then the layers of material of the block 

 under the area have been elevated by an amount approximately nine- 

 tenths of the thickness of the eroded material; that is, each layer 

 of the block will have been brought closer to the surface of the earth 

 by that amount, As a result of this uplift of material of the block 

 under the mountain area, each layer will be raised to a zone which is 

 colder by an amount equal to the change in temperature from the 

 zone previously occupied to its subsequent location. If the vertical 

 movement has been five miles and should the temperature gradient be 

 about 50° C. per mile, then the material of each layer will have its 

 temperature decreased by about 250° C. This change in temper- 

 ature, however, will probably occur at a much slower rate than the 

 rate of uplift of the material of the block during the erosion. 

 When the material finally assumes the new and much lower tempera- 

 ture it will have undergone a thermal contraction, but this thermal 

 contraction cannot lower the surface more than about 2,500 feet. 

 There must occur an additional increase in density and shortening 

 of the column from some physical or chemical process. 



This process must be started by the lowering of the temperature 

 of the material throughout the column, and it continues during the 

 entire period of the subsequent sedimentation. This seems to be a 

 logical assumption in view of the conditions existing in a sedimen- 



