328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



We know that there has been a tremendous amount of erosion and 

 sedimentation during the present era, which is called the sedimentary 

 age of the earth. It is certain that the earth's surface was irregular 

 at the time that sedimentary rocks began to be formed, for without an 

 irregular surface there could have been no running water, and with- 

 out running water there could have been no erosion and sedimenta- 

 tion. Of course, no one knows whether the amount of water on the 

 earth has been constant or variable, but it is reasonably certain that 

 land has been exposed above the waters of the ocean for about 

 a billion and a half years. This is an estimate that is frequently used 

 by students of the earth, and it seems to be generally accepted as of 

 the order of magnitude of the period of time that has elapsed since 

 the formation of the first sedimentary rocks. 



Geologists tell us that practically all of the exposed areas of the 

 earth have at some time in the geological past been beloAV sea level. 

 These areas are now at varying distances above sea level and, hence, 

 their change in elevation, with respect to sea level, must have been 

 due to an actual lifting up of land areas rather than a decrease in 

 the amount of water of the earth. If the latter had been the cause 

 for the changes in elevation, there would be uniformity in the eleva- 

 tions of exposed strata. 



The isostatic investigations indicate that the solid or rigid material 

 of the earth extends only to a depth of approximately 60 miles below 

 sea level. Some investigators are of the opinion that the depth to 

 which the solid rock extends is very much smaller than that. The 

 interior of the earth acts as if it were plastic to long-continued 

 stresses. The earth has an outer shell which rests upon a plastic 

 interior. A disturbance of the isostatic equilibrium leads to hori- 

 zontal and vertical changes in the earth's surface. Some areas go 

 down under the weight of sediments and other areas which have been 

 undergoing erosion for long periods of time increase in elevation. 

 There is also a rising up of material that was once below sea level 

 and a sinking down of areas that were once standing high above 

 sea level. 



These and other known facts regarding the earth arc the basis for 

 the interpretation of the processes which have shaped its surface. 



There have been many theories advanced as to why the earth has an 

 irregular surface. Such theories may be considered as mere guesses, 

 for no one can reproduce to-day the forces, resistances, and tempera- 

 tures that must have been involved when the earth came into being 

 or when its surface was changed from one of fairly uniform eleva- 

 tion to one which has the great differences in elevation that are seen 

 to-day. 



Mineralogists tell us that the continents are underlaid by granite, 

 and that granite is absent from the crust under the ocean. Granite 



