238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



It has been found that there is no difference in the isostatic con- 

 dition from one place on the earth to another, nor is there any 

 difference in this condition between blocks under elevated areas and 

 those of little or no elevation. In other words, the isostatic con- 

 dition is entirely universal, and what may be called the isostatic 

 gravity anomalies, or the difference between observed and com- 

 puted values of gravity, and the isostatic deflections of the verti- 

 cal show no relation whatever to the elevation of the. ground on 

 which the various values of gravity or deflections of the vertical 

 may be determined. 



Since we find all blocks of the earth's crust in equilibrium now, 

 it appears certain that a like equilibrium existed in past geological 

 ages or periods. If this be true, as it seems most reasonable to 

 assume, we must change considerably some of our ideas as to the 

 causes of the yielding of the material of the earth's crust and also 

 change our views as to the direction in which much of this yield- 

 ing has occured. 



EROSION AND SEDIMENTATION IN LIGHT OF ISOSTASY. 



All elevated areas are continually losing material from the action 

 of water and air. This material is carried by these elements down 

 to regions which are lower. A mountain system loses material in 

 vast quantities which is transported by streams and rivers to the 

 valleys and the edges of the ocean. Undoubtedly, the isostatic bal- 

 ance of the blocks of the earth's crust is disturbed by this trans- 

 fer of material. The block from which material is being eroded 

 will become lighter than normal, and the block on which the sedi- 

 ments are placed will become heavier. It is practically certain that, 

 as a result of this disturbance of the balance, the earth's crust yields 

 under the sedimentary area, causing a downwarping and an outflow 

 of material at the bottom of the sedimentary block towards blocks 

 from which material has been eroded. The effect of the horizontal 

 surface movement towards the sedimentary area will result in other 

 material entering at the bottom of the block from which material 

 is eroded. The addition of this material to the bottom of the eroded 

 columns results in a rising of that column. 



As a consequence of the transfer of material at the surface of the 

 earth, there is a sinking of one block, the rising of another, and a 

 horizontal sub-crustal transfer of material from the first towards 

 the second. 



We can thus see that there is a tendency for any area to be main- 

 tained in its elevation by the isostatic adjustment. There would be 

 no change in the elevation of the surface, as a result of the transfer 



