THE STRENGTH OF THE EARTH'S CRUST 2ii. 



confirmatory of those reached in Parts I and II. In addition, how- 

 ever, it appears that there is a regional departure from isostasy of 

 two orders of magnitude. Loads under the mean value, giving 

 anomalies below 0.018 to 0.020 dyne and estimated to be equiva- 

 lent to about 750 feet of rock, can be carried over regions of irregu- 

 lar boundaries ranging up to from 1,000 to 2,000 km. across. Over 

 such a broad region the anomalies are of one sign except for some 

 smaller well-defined sub-areas of high anomaly within them which 

 may or may not have the same sign. These smaller areas give a 

 higher order of stress magnitude and are of more restricted dimen- 

 sions, being measured in hundreds of kilometers. They range in 

 magnitude of anomaly to several times the value of the mean and 

 the equivalent radii of their areas probably average 100 to 200 km. 

 The- deflection residuals show by the limits of the areas of like 

 sign that the regional variations of gravity anomahes of this 

 areal magnitude extend over the whole country, but where the 

 amounts of the local anomalies are less in value than the mean 

 they are largely masked on the contour map of gravity anomalies 

 (Fig. 5), because of their superposition upon the broader areas. 

 Presumably a multiplication of the gravity stations would bring 

 them to light as undulations in the contours which show the 

 regional departures. 



A final conclusion on the subject of the variable vertical distri- 

 bution of mass must, however, be deferred until consideration has 

 been given to a hypothesis advanced by Gilbert in his recent paper, 

 that heterogeneities of mass below the zone of compensation may 

 be the cause in major or minor part of the apparent departures from 

 isostasy. This is a subject too large to be considered in this third 

 part of the present article, but it is planned to investigate it in 

 Part V by a method of graphic analysis devised for determining 

 the depth of excesses or deficiencies of mass. 



THE SPECIFIC GRAVITY OF ROCKS 



For a knowledge of the variations of density likely to occur in 

 rocks it is important to know the range in specific gravities shown 

 by the common rock types. The following figures, except those for 

 shale, are taken from Pirsson's Rocks and Rock Minerals: 



