OUR PRESENT KNOWLEDGE OF ISOSTASY 439 



of the distance of the attractive mass from the disk; therefore, if 

 we should have in Canada a station on an extensive area of pre- 

 Cambrian formation where the material is of uniform thickness, 

 and if this material were compensated for by a deficiency of material 

 below it, then the compensation would have an effect which 

 would practically counterbalance the effect of this denser material 

 which is near the surface. 



In India there are only eight stations on the pre-Cambrian 

 formation, six of them having positive anomalies and two negative. 

 But the mean with regard to sign of the anomalies is nearly zero. 

 It may be possible that the small number of stations on this forma- 

 tion in India prevents the stations there from showing the same 

 relation to the formation that we have in the United States. It is 

 worthy of note that each of the areas of the pre-Cambrian forma- 

 tion in the United States on which stations are located is rather 

 small in horizontal extent. If we should have a pre-Cambrian 

 formation 10,000 feet thick under a station with a density of the 

 rock 10 per cent above normal, and if the formation extended 

 10 km. in all directions from the station, the effect of the increased 

 density would be to increase gravity by +0.029 dyne. If this 

 extra material were completely compensated for and the compen- 

 sation were distributed uniformly to a depth of about 114 km., 

 the negative effect of the compensation would be —0.003 dyne. 

 The resultant would be +0.026 dyne, which is about the size 

 of the average pre-Cambrian anomaly in the United States. 



It was found that the anomalies at stations on the Cenozoic 

 formation had a tendency to be negative both in the United States 

 and in India. There were only two stations in Canada on this 

 formation and they were both negative. It is probable that the 

 reasoning employed above in regard to the pre-Cambrian anomalies 

 will apply to the Cenozoic anomalies. The density of the material 

 of this formation is in general about 5-10 per cent less than normal, 

 and the presence of this light material near the station should have 

 a greater effect 6n the value of gravity than the compensation of 

 this material, if any, which would be lower down in the lithosphere. 



If the Cenozoic formation should be of great horizontal extent 

 and of uniform thickness, the effect of material of this formation 



