12 



EARTH FEATURES AND THEIR MEANING 



Anger re 



the land masses into northern and southern portions along the 

 course of a great circle which makes a small angle with the earth's 

 equator. Thus the western continent is nearly divided by a 

 mediterranean sea, the Caribbean, and the eastern is in part 

 so divided by the separation of Europe from Africa. 



The figure toward which the earth is tending. Thus far in 

 our discussion of the earth's figure we have been guided entirely 



by the present dis- 

 tribution of land and 

 water. There are, 

 however, depres- 

 sions upon the sur- 

 face of the land, in 

 some cases extend- 

 ing below the level 

 of the sea, which are 

 not to-day occupied 

 by water. By far 

 the most notable of 

 these is the great 

 Caspian Depression, 

 which with its ex- 

 tension divides cen- 

 tral and eastern Asia 

 upon the east from Africa and Europe upon the west. This 

 depression was quite recently occupied by the sea, and when 

 added to the present ocean basins to indicate depressions of the 

 lithosphere, it shows that the earth's figure departs from the 

 standard spheroid in the direction of the form represented in 

 Fig. 3. This form approximates to a tetrahedron, a figure bounded 

 by four equal triangular faces, here with symmetrically truncated 

 angles. Of all regular figures with plane surfaces the tetrahedron 

 has the smallest volume for a given surface, and it presents more- 

 over a reciprocal relation of projection to depression. Every 

 line passing through its center thus finds the surface nearer than 

 the average distance upon one side and correspondingly farther 

 upon the other (Fig. 4). 



Astronomical versus geodetic observations. Confirmation of 

 the conclusions arrived at from the arrangement of oceans and 



FIG. 3. The form toward which the figure of the earth 

 is tending, a tetrahedron with symmetrically truncated 

 angles. 



