SHAPING THE EAKTH 



By William Bowie 

 V. S. Coast and Qeodetie Survey 



THE CRUST OF THE EARTH 



It is generally recognized that the earth has had a surface of solid 

 material for something like a billion and a half years. At the begin- 

 ning of this time the earth's surface was irregular and there have 

 been vertical and horizontal changes occurring continuously during 

 this long interval. These changes have been due to erosion and sedi- 

 mentation and to forces which are acting on the materials forming 

 the outer 50 or 100 miles of the earth. 



If the earth's material were in a liquid or highly plastic condition, 

 and if there were no rotation, its surface would be a true sphere. 

 With such a body undergoing rotation the surface would be a spher- 

 oid. It has been found by geodetic measurements that the shape of 

 the mean sea-level surface approximates very closely a true spheroid. 

 The deviations between the spheroid and the water surface, or geoid, 

 are probably not greater than 100 meters. These forms are, of course, 

 due to the continuous gravitational attraction of the particles of the 

 earth for each other. The earth's surface is irregular because of the 

 presence of material of different densities near the earth's surface. 

 Under the continents the densities are less than they are for the mate- 

 rial under the oceans. There is rigidity in the outer portion of the 

 earth for otherwise there would be a slumping down of the high areas 

 and the moving material would fill up valleys and ocean basins and 

 bring the earth's surface to a true spheroid. 



FORMATION OF OCEANS AND CONTINENTS 



One of the most interesting problems of geology involves the 

 formation of oceans and continents. Some geologists will say that 

 this is a subject that need not be considered for we may accept oceans 



1 Presidential address delivered before the Washington Academy of Sciences, Jan. 15, 

 1931. Reprinted by permission, with author's revision, from Journal of the Washington 

 Academy of Sciences, vol. 21, No. 6, Mar. 19, 1931. 



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