Bowie — Recent Gravity Work in the United States. 113 



anomaly and the topography. The nearly complete disappear- 

 ance of the association between anomalies and topography is a 

 proof that the condition called isostasy exists, and that the 

 assumption that the compensation is complete and uniformly 

 distributed with respect to depth from the surface to a depth 

 of 113 - 7 kilometers is a close approximation to the truth. This 

 value for the limiting depth of compensation was used instead 

 of 122 kilometers (the latest value) as the computations were 

 begun before the latter value was available. 



The facts and conclusions stated above are for the 56 gravity 

 stations in the United States used in the reduction made by 

 Mr. Hayford in 1909. He and the speaker have nearly com- 

 pleted an enlargement of the first investigation with the addi- 

 tion of 33 stations, making 89 in all. It is expected that the 

 results of this investigation will be published by the Coast and 

 Geodetic Survey during this year. This investigation will 

 verify the conclusions arrived at from the first one. Certain 

 new features have been added. One is a study to show whether 

 the theory of local distribution of the compensation is nearer 

 the truth than what may be considered regional distribution. 

 Another is an analysis of the anomalies to show whether there 

 is any connection between the size and sign of the residual and 

 the geologic formation. Though the number of stations is too 

 small to justify the statement that there is a positive connec- 

 tion, yet the evidence is strong that there is a relation. This 

 is especially the case with regard to the oldest and most recent 

 formations. 



It has been found that the results from the gravity investi- 

 gations are in almost perfect harmony with those of Hayford's 

 investigations of the figure of the earth and isostasy. Those 

 who have read his second publication may remember the illus- 

 tration on which are shown by ovals certain areas of excessive 

 and defective mass, as indicated by the anomalies of the 

 deflections. Gravity stations have been established in many 

 of these areas and in each case the gravity anomaly by the 

 Hayford method is of the same sign as that indicated by the 

 deflections. 



To sum up — the work done by the Coast and Geodetic Sur- 

 vey proves conclusively that a close approximation to the con- 

 dition of complete isostasy exists. The introduction of the 

 theory of isostasy into the reduction of gravity observations 

 has made the anomalies very small. Further investigations of 

 deflections of the vertical and of gravity observations may show 

 how the compensation is distributed and how nearly it is com- 

 plete. With increased knowledge of the distribution of com- 

 pensation the figure and size of the earth can be more 

 accurately determined. 



United States Coast and Geodetic Survey, Washington, D. C. 



