146 JOSEPH BARRELL 



It is an important conclusion, established by geodetic evidence, 

 that the ocean basins are underlain by heavier matter than that 

 beneath the continental platforms; the tendency through geologic 

 time for the continents to rise relatively to the oceans may be 

 correlated with this difference in density and the lightening of the 

 land areas by the progressive erosion of the land surfaces. It is 

 believed that the rejuvenative movements are in the direction of 

 isostatic equilibrium. Fortunately for land-dwelling vertebrates, 

 the crust is too weak for readjustment to be deferred until after the 

 erosion of the lands, begun by the subaerial forces, shall have been 

 completed by the sea. 



But the power of geodetic research does not cease with the 

 establishment of this cause of the maintenance of the differential 

 relief between land surface and ocean floors. Beneath the surface 

 of the continents it reveals heterogeneities of density and measures 

 them against the more or less local relief above. To the extent to 

 which areas of lighter or denser matter do not correspond to pro- 

 portionately higher or lower rehef, real strains either upward or 

 downward are shown to exist through the crust. Over areas of 

 plains which have not suffered much change for geologic ages, 

 geodesy may thus reveal the existence of large crustal strain. On the 

 contrary, in regions of mountainous rehef, although the individual 

 mountains are sustained by rigidity and bring local strains upon the 

 supporting basement, geodetic study may show that there is close 

 regional compensation of density balanced against rehef, obliterat- 

 ing with depth the stress differences due to topography. These 

 methods of research are thus capable of attacking the problem of 

 the amount and direction of vertical strain existing in the crust 

 under any part of the land surface and, to a lesser degree of accuracy, 

 the crust beneath the sea. The breadth of the individual areas 

 which depart from equihbrium in one direction may constitute also 

 a vital part of the problem. 



But although these are fields of research open to the geodesist, 

 they are cultivated with much labor. The position of many sta- 

 tions on the surface of the earth must be determined by astronomic 

 observations to within a fraction of a second of arc. Then a 

 triangulation network, continent-wide, ties these together and shows 



