Uplift and Subsidence. 9 



we considered the topography out to about 100 miles. 

 The topography for 100 miles farther has little or no 

 effect on the station because of the great distance and the 

 fact that the topography is nearly in the same horizon as 

 the station, thus making the vertical component very 

 small. We may conclude, I think, that the second method 

 gives the better result. By it the average gravity ano- 

 maly is about 13 per cent of what it would be on a rigid 

 earth. This is so close to the ten per cent obtained by 

 Hayford as the relation between the isostatic deflection 

 and the rigid deflection that I shall, in the remainder of 

 this paper, adopt ten per cent as the relation between 

 isostatic and rigid anomalies, both deflection and gravity. 



Eelation between isostatic anomalies and areas of erosion. 



It has been found that there is no definite relation 

 between the sign and size of the isostatic gravity anomaly 

 and areas of erosion. There may, of course, be areas of 

 erosion where the gravity anomaly tenSs to be negative 

 or to be positive, but we may say that no definite relations 

 can be found that are general in their application. 



Relation hettveen isostatic anomalies and recent geological 



formations. 



We have found a very definite relation between areas 

 of recent sedimentation and gravity isostatic anomalies, 

 and this definite relation applies to stations on all the 

 sub-divisions of the Cenozoic formation.. 



For a number of years the negative gravity results 

 along the coasts and in inland areas of recent sedimen- 

 tation were thought to be an indication of a lack of isos- 

 tatic equilibrium in those areas. It was early found, in 

 the investigations by the writer, that there is a definite 

 relation in sign between the gravity anomalies and the . 

 Cenozoic formation. In the report^ on gravity investi- 

 gations, published in 1917, the writer arrived at the 

 conclusion that the Cenozoic anomalies were negative 

 because the material of this formation, extending some 

 distance below sea-level, is much lighter than normal. 

 In most cases it is probably more than 10 per cent less 

 than normal. It is apparent that if we have very light 



^Investigations of gravity and isostasy, spec. publ. 40, U. S. Coast and 

 Geodetic Survey, 1917. 



