316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



These departures from the normal are known as "anomalies." 

 They may be either positive, when the gravity is above the normal, 

 or negative, when it is below. The anomalies have been studied 

 with great care and in great detail, especially by Hayford and 

 Bowie in the area of the United States, who have invoked the 

 theory of isostasy to account for them. This explanation is satis- 

 factory to a very large extent, but, as Iddings says, " There remain 

 anomalies of density which need to be accounted for." On the 

 whole, however, it appears that the theory of isostasy " obtains for 

 the major features of the earth's surface." It may be suggested 

 here (though the matter can not be discussed) that the discrepancies 

 may be due, in part at least, to the fact that the geodesists have taken 

 account of the rock densities only of those portions very near the 

 surface, mostly soils and sedimentary rocks, and have neglected the 

 deeper-lying portions. The average density assumed by geodesists 

 for the surface rocks is 2.67, while, as we have seen, that of the 

 igneous rocks of the earth's crust is 2.77 or 2.75. 



It will be seen that the idea of the continental masses being com- 

 posed of light material while the ocean floors are of heavy is by no 

 means new. Up to the present, however, there has been no quanti- 

 tative verification of this, except for the few figures covering limited 

 areas given by Iddings. The data given above, with the graphs, 

 therefore, are of special interest as furnishing a first approximation 

 to a knowledge of the actual densities of the various portions 

 of the earth's crust. It is evident that they are, on the whole, and 

 even in considerable detail, quite in harmony with the theory of 

 isostasy. Indeed, based, as these figures are, on a large number of 

 data (the largest by far yet available) from all parts of the earth, 

 and showing such a complete harmony between average density and 

 average elevation everywhere, they may fairly be said to be more than 

 coincidental, and to constitute almost a conclusive proof of the gen- 

 eral validity of the theory of isostasy. 



One further point of agreement may be mentioned. In figure 6 

 is given a map of the United States reproduced by Barrell 44 from 

 Bowie, showing the distribution of the gravity anomalies over the 

 United States. Let us compare this with the description of the 

 comagmatic regions of the United States (fig. 2). In figure 6 the 

 dotted areas are those of positive anomaly (excess gravity), while 

 those not clotted are of negative anomaly (deficient gravity). 



In the extreme northeast is a small area of positive anomaly about 

 the Adirondacks, which corresponds with the small comagmatic out- 

 lier there of the Canadian Shield, of which the rocks are above 

 the average in density. The greater part of Maine, with its 



"Barrell, J., Jour. Geol ., xxii, p. 153, 1914. 



