10 GEOLOGY. 



equator being sufficient to cause the specified amount of bulging there. 

 Computations seem to indicate that the accommodation is very nearly 

 what would take place if the earth were in a liquid condition, from 

 which the inference has been drawn that it must have been in that 

 condition when it assumed this form, and must have continued essen- 

 tially liquid until it attained its present rate of rotation, since, if the 

 earth once rotated at a much higher speed, the flattening at the poles 

 and the bulging at the equator must have been correspondingly greater. 

 It is thought by others, however, that the plasticity of the earth is 

 such that it would at all times assume a close degree of approximation 

 to the demands of rotation, even if the interior were in a solid condition. 

 By still others it is thought that the contraction of the earth has tended 

 to accelerate the rotation about as much as the tides have tended to 

 retard it, and that it has undergone little change of form. 



Irregularities. — It is only in a general view, however, that there 

 is a close approximation to a perfect spheroidal surface. In detail 

 there are very notable variations from it. Geodetic surveys seem to 

 have shown that the equatorial diameters are not all equal, even when 

 the measurements are reduced to sea-level, but research along this 

 line has not reached a sufficient stage of completeness to permit satis- 

 factory discussion. It is, however, highly probable that the ocean surface 

 as well as the average land surface is warped out of the perfect spheroidal 

 form to some notable degree. This is very likely due to inequalities in 

 the density of the earth's interior. The fact that the larger portion 

 of the water is gathered on one side of the globe, while the land chiefly 

 protrudes on the opposite side, is very possibly due to unequal specific 

 gravity in the interior of the earth. 



The most obvious departure from a spheioidal form is found in the 

 protrusion of the continents and in the sinking away of the earth 

 surface under the oceans. As these inequalities present themselves 

 to-day, they are known as continental platforms and ocean basins. 

 These do not correspond accurately with the present land and water 

 surfaces.. About the continental lands there is a submerged border 

 extending some distance out from the shore, and constituting a sea- 

 shelf beyond which the surface descends rapidly to the great depths of 

 the ocean. This slightly submerged portion, known as the continental 

 shelf, belongs as properly to the continent as the adjacent low lands 

 which are not submerged. The submergence of the edge of this shelf 

 at present is usually about 100 fathoms, so that if the upper 600 feet 



