THE STRENGTH OF THE EARTH'S CRUST 



221 



•2.67 and the lower cylinder, V, is 2 per cent heavier. Let these 

 abnormalities be limited areally to the cylinder. This is a departure 

 in density of o. 054, 15 . i times the density o . 00357. The anomalies 

 will be as follows: 



TABLE XII 

 Anomalies Due to Irregular Vertical Distribution of Density 



It is seen from this tabulation that, first, irregular superposed 

 but balanced positive and negative distributions of density up to 

 distances as large as the radii of the areas of grouped residuals 

 could produce at least a considerable part of the anomalies; or, 

 second, actual departures from isostatic equilibrium with the 

 resultant strain on the crust could produce them; or, third, a 

 combination of the two. In the second case, as Hayford and 

 Bowie show,^ the anomalies could result from a layer a few miles 

 thick adjacent to the station and of very abnormal density; or 

 from deep and regional masses of great volume, but departing 

 only slightly from the mean density. The choice between these 

 several alternatives, or the degree to which they co-operate, must 

 be investigated under the following topics. 



RELATIONS OF ANOMALIES TO EXPOSED GEOLOGIC FORMATIONS 



The latest data given by Bowie on this subject are shown in 

 Table XIII (p. 222):^ 



These figures of course are not to be regarded as of high pre- 

 cision, as may be seen by comparing the earlier and later results. 



'Op. ciL, Pp. 108-11. 



' " Some Relations between Gravity Anomalies and the Geologic Formations in 

 the United States," Am. Jour. Sci. (4), XXXIII (1912), 237-40. 



