OUR PRESENT KNOWLEDGE OF ISOSTASY 441 



with the effect of the deficiency in density in the materials close to 

 the surface and under the station. 



It seems probable that we may be able to predict with some 

 accuracy a gravity anomaly on a Cenozoic or pre-Cambrian forma- 

 tion when we know the latitude and elevation of the point of obser- 

 vation and make a correction for the topography of the world and 

 its compensation and apply a correction for the negative or positive 

 attraction of the deficiency or excess of matter in the Cenozoic or 

 pre-Cambrian formation. This, of course, is with the provision that 

 the approximate depth and the horizontal extent of the material of 

 these formations in the immediate vicinity of the station are known. 

 It is also possible that where a station is located on a Cenozoic 

 formation and has a positive anomaly the Cenozoic material is 

 of slight thickness and is underlaid by pre-Cambrian or other extra 

 dense material. 



The stations in the United States on Paleozoic formations show 

 a tendency to have negative anomalies. The mean anomaly with 

 regard to sign is —0.009 dyne. The Mesozoic stations have a 

 tendency to be positive with a mean anomaly with regard to sign 

 of +0 . 01 1 dyne. There were so few stations on intrusive and effu- 

 sive formations that it is believed that no definite results were ob- 

 tained from a study of them. There were enough stations in the 

 pre-Cambrian, Cenozoic, Mesozoic, and Paleozoic formations to 

 enable one to state rather definitely that stations on any one of 

 them have a decided tendency to have anomalies of a certain sign. 

 There cannot, however, be any relation between the sign of the 

 gravity anomaly and the density of the Paleozoic or the Mesozoic 

 material, for, in general, the density of the material of those two 

 formations is about normal. 



Where the surface density is subnormal and the gravity anomaly 

 is positive, it may be possible that there is denser material somewhat 

 lower down in the lithosphere if the size of th$ anomaly is large. 

 By large is meant somewhat above the average size of the anomaly 

 without regard to sign. There are a number of places in the 

 United States in which borings have disclosed the presence of 

 crystalline rocks at varying depths below the surface where the 



