190 abstracts: geodesy 



given. Id fact, all the data are given that are necessary for making 

 reductions at any gravity station in the world. 



The procedure at each of the gravity stations was to compute the theo- 

 retical gravity for the latitude of the station in question by the Heimert 

 formula, correct this for the elevation of the station above sea level and 

 then apply the correction for the topography of the world and its compen- 

 sation. The resulting theoretical value is then compared with the actu- 

 ally observed value of gravity at the station. The difference between the 

 computed and observed values is the anomaly which indicates a depar- 

 ture at that station from the assumed density of the earth's surface and 

 of the computed excesses and defects of density in the crust to the depth 

 of compensation. 



A comparison is made of the new method anomalies in size and sign, 

 with the anomalies given by the free air and Bouguer methods and the 

 result is in practically every case in favor of the new method. 



The 89 stations in the United States were arranged in groups with 

 reference to their relation to topography. The groups are: 16 coast 

 stations; 18 stations near the coast (within 325 kilometers); 27 conti- 

 nental stations not in mountainous regions; 16 stations in mountainous 

 regions, below the general level; and 12 stations in mountainous regions, 

 above the general level. The mean without regard to sign of the anoma- 

 lies at 87 stations (two stations not considered) is 0.017 dyne. For the 

 five groups mentioned above the corresponding means are 0.017, 0.020, 

 0.018, 0.012, and 0.014 dyne, of which no one is much above the general 

 mean of all. 



The means with regard to sign for the five groups are: — 0.004, 

 + 0.002, + 0.002, - 0.002, and + 0.003 dyne. The general conclu- 

 sion from the examination is that the anomalies by the new method 

 show no relation to the topography either in sign or average magnitude. 

 It is shown on the other hand that there are decided relations between 

 the anomalies by the two older methods of reduction and topography. 



It was found that there appears to be a relation between the surface 

 geologic formation and the size and sign of the new method anomalies. 

 The mean of the anomalies at seven stations on Pre-Cambrian formation 

 is + 0.019 dyne, with regard to sign, and 0.026 dyne, without regard to 

 sign. This indicates an excess of material in the crust in the vicinity of 

 these stations. The means with and without regard to sign of the anoma- 

 lies at 20 stations in the Cenozoic are — 0.011 and 0.021 dyne, respec- 

 tively. This appears to indicate a deficiency in mass in the crust at 

 these stations. It is shown that these anomalies are probably caused by 



