344 ANNUAL REPORT SMITHSONIAN mSTlTtJTION, 1931 



outlined above would seem to show how this oscillation can take 

 place. 



We have seen from the above analysis what forces are being ex- 

 erted on the materials of the earth. The movement downward of the 

 crust under the sediments causes a movement of subcrustal material 

 back toward the region from which the sediments were derived. The 

 horizontal movement to restore the balance must occur below and not 

 within the crust. What the effect is of this horizontal movement of 

 subcrustal material on the surface configuration of the earth between 

 the areas of erosion and sedimentation we do not know. Some think 

 that perhaps much of the wrinkling of the earth's surface is due to 

 this subcrustal flow. I am inclined to think that the movement of 

 subcrustal material is so small in extent that there can be little effect 

 of it on the surface above the crust about 60 miles in thickness. I do 

 not think there is anything like a river of material flowing from the 

 region below the sedimentary area to the erosion area. It is more 

 likely that the moving material involves a large volume, and any 

 portion moves only a very short distance. I believe that this move- 

 ment of subcrustal material, which is a part of the isostatic adjust- 

 ment, exerts only a minor influence on those portions of the surface 

 of the earth that lie between the areas of sedimentation and erosion. 



From the above reasoning there appear to be four definite causes of 

 changes in the elevation of surface areas aside from the direct effects 

 of erosion and sedimentation: First, the depression of the crustal 

 material under an area of sedimentation; second, the moving upward 

 of crustal material to restore the balance under an area of erosion ; 

 third, the expansion of crustal material which has been depressed 

 by great loads of sediments; fourth, the contraction of the earth's 

 crust and the sinking of the surface under an area of erosion. These 

 must be the causes of many of the earthquakes of the world, although 

 it would not be safe to assert that these are the only causes of earth- 

 quakes and surface movements. 



Many geologists do not give as much weight as I do to effects of 

 sedimentation and erosion on changes in the configuration of the 

 earth's surface. Prof. C. K. Leith, in his splendid book entitled, 

 " Structural Geology," published in 1923, tells us that isostasy and 

 the maintenance of isostatic equilibrium are minor causes of struc- 

 tural changes. He expresses his views as follows : 



So far as it is possible to generalize from this vague state of knowledge, 

 it may be said that geologists are at present inclined to give principal place 

 to changing rate of rotation and to the shrinkage of the earth, due to heat 

 transfer from the interior outward, whether they go back to the nebular or 

 planetesimal hypothesis of the origin of the earth ; that metamorphism and 

 chemical changes, vulcanism, and forces tending to maintain isostatic equilib- 

 rium are regarded as subordinate or contributory causes, or perhaps as special 

 and local expressions of the more basic causes first indicated. 



