704 



SCIENCE 



[N. S. Vol. XXXIX. No. lOll 



there is no large area which departs much 

 from the perfect isostatie state. 



It has been held by some^^ that the crust 

 of the earth within the area of the United 

 States is at all places in a state of perfect 

 isostasy and that the gravity anomalies 

 are caused by the investigators' erroneous 

 distribution of the compensating deficiency 

 of mass. This, no doubt, is true to a cer- 

 tain extent, but no rational distribution 

 will account for the larger anomalies 

 except upon the theory that there are local 

 a-etual departures from the perfect isostatie 

 state. 



Earlier in this paper it was stated that 

 there were developed some relations be- 

 tween the size and sign of the gravity 

 anomalies and the geological formation. 

 The stations established on the oldest rocks 

 show an average anomaly of + 0.024 dyne, 

 that is they indicate too much mass in the 

 earth's crust, while the most recent forma- 

 tions and the effusive and intrusive rocks 

 give average negative anomalies which vary 

 from —0.005 to —0.015 dyne. These 

 show deficiencies of mass. It is not clear 

 whether these anomalies are due to local 

 deviations from the adopted mean surface 

 density of the earth (2.67) or to actual 

 departures from a state of complete isostasy. 



The following table shows a summary of 

 the gravity anomalies arranged in the sev- 

 eral geological groups. 



A stratum of material 100 feet in thick- 

 ness, and of indefinite extent will cause 

 an attraction of 0.0030 dyne. Therefore, 

 the average anomaly 0.019 dyne would 

 correspond to a stratum 630 feet in thick- 

 ness. The average elevation of the United 

 States is about 2,500 feet. Therefore, the 

 isostatie compensation may be considered 



12 ' ' Interpretation of Anomalies of Gravity, ' ' 

 by Grrove Karl Gilbert, Part C of Professional 

 Paper 85, Contributions to General Geology, 1&13, 

 IT. S. Geological Survey. 



to be about 75 per cent, complete on an 

 average for stations in this country. But 

 the average gravity anomaly with regard 

 to sign is 0.000 dyne, or nearly so, there- 

 fore it is safe to conclude that the area of 

 the United States as a whole is in a state 

 of perfect isostasy. 



A more exact knowledge of the precise 

 location with respect to depth of the com- 

 pensating deficiency or excess of mass 

 would be of very great interest and value. 

 This is particularly true in the case of the 

 areas where there are stations with large 

 gravity anomalies. A preliminary study 

 by the Survey will be made of this question 

 in the near future. For this purpose the 

 value of gravity and the deflections of the 

 vertical in the vicinity of the anomalous 

 areas will be used jointly. The prelimin- 

 ary studies may show whether much is to 

 be expected from future investigations 

 after much more gravity and deflection 

 data are available. 



An attempt will be made shortly to cor- 

 rect the deflections for the effect of the 

 excesses and deficiencies of mass as shown 

 by the gravity anomalies. It is possible 

 that some of the unaccounted for portion of 

 the deflections may be eliminated. The 

 ideal condition would be to have only such 

 residuals of the deflections as are caused 

 by an error in the adopted figure of the 

 earth, for then a new figure might be com- 



