26 THE FLOOR OF THE OCEAN 



sible to see how far the wavy geoid departs from the standard 

 ellipsoid or standard spheroid. We may think of the geoid as 

 made of humps above and hollows below the standard surface. 

 The hump-and-hollow waviness is clearly caused by irregulari- 

 ties in the distribution of matter in the earth's body. Under or 

 near each geodetic station located on a geoidal hump there is 

 excess of attracting matter; under or near each station located 

 on a geoidal hollow there is defect of attracting matter. 



In average the continents project a half-mile above sealevel 

 and in average the bottom of the deep sea is 2.5 miles below 

 sealevel. If the ocean water, with its density of 1.03, were con- 

 verted into continental rock, with density of 2.7, the mean 

 continental surface would stand about two miles above the sur- 

 face of that condensed ocean. Each of the projecting continental 

 masses, covering millions of square miles in area and two miles 

 in thickness must exert a strong horizontal pull on the plumb- 

 line. If that continent represented an excess of matter in the 

 corresponding sector of the earth, plumb-bobs suspended on all 

 sides of the continent would be pulled sideways, toward the 

 central part of the land, and pulled so much that the angles of 

 the pull could be easily measured. The inward deflection of 

 the plumb-line would mean a continent-wide hump on the 

 geoid, for at any point in the geoid this surface must be rigor- 

 ously at right angles to the local plumb-line. The geoidal 

 hump should culminate near the center of the continent. See 

 Figure 12, in which the broken line represents this arching of 

 the geoid over the continent. 



Moreover, the theory of gravitational attraction shows that 

 the mean level of an ocean between two such continental bodies 

 of excess matter would have to be decidedly hollowed below 

 the standard spheroid. For example, if the Americas and the 

 lands of the Old World were extra masses, there should be an 

 easily demonstrable hollow in the geoid over the Atlantic 



