336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



masses slumping down and flowing into the ocean areas to fill up the 

 basins. This strong material extends, according to geodesists, ap- 

 proximately 60 miles below sea level. Below that the material must 

 be lacking in strength and rigidity. It must yield to forces without 

 breaking. As great masses of material are moved over the earth's 

 surface the balance of the crust is disturbed. The extra load caused 

 by sediments must push down the crust beneath and this must force 

 the subcrystal material to move sidewise and some of it to push up the 

 crust from where the eroded material came. The earth's crust is like 

 a sheet of ice on a pond or on the Arctic Ocean. The crust lies quietly 

 on the interior part of the earth until something happens to disturb 

 the equilibrium. Although the crust of the earth is composed of 

 strong material, the strength is finite, surely not great enough to with- 

 stand the weight of the tremendous loads that have been shifted on 

 the earth's surface. It is, however, strong enough to maintain the 

 irregular surface of the earth just because of the floating principle. 



Earthquakes have occurred probably in all parts of the earth. 

 One can not make an accurate estimate of the maximum size of 

 the portion of the earth's crust in which, throughout geological 

 time, no earthquakes have originated, but we see ail about us evi- 

 dences of uplift or subsidence of the earth's surface. Each con- 

 tinent has above sea level much sedimentary rock that must have 

 been formed below tidal waters. These rocks in many cases are 

 much tilted, curved, broken, and crushed. It is reasonably certain 

 that there has been an uplift of the earth's surface rather than a 

 decrease in the amount of ocean waters to cause these exposures. 

 The best evidence that they have been pushed up is the fact that 

 strata laid down in salt water in horizontal positions are now tilted 

 at various angles from the horizontal. Then again, the same strata 

 exposed in a number of widely separated places are found at different 

 elevations above sea level. This, it seems, is an indication that there 

 has been an actual uplift of the earth's surface. Every one who 

 has engaged in mining operations knows of the tremendous amount 

 of faulting that has occurred in the rocks. A coal seam will be 

 followed for a certain distance and then it gives out. Later the 

 same seam of coal may be found at a higher or lower elevation. 

 The many fractures that are found in mines and at the earth's surface 

 lead one to the very definite conclusion that there has been much 

 shifting of material in the geological past. Each one of these shifts, 

 or changes, where a fracture has occurred, has probably caused an 

 earthquake. 



The earth may be classified as a yielding body. It should not be 

 classed as a failing structure. A soapbubble or a glass ball, when 

 subjected to stresses greater than its strength, will collapse, but it is 



