THE STRENGTH OF THE EARTH'S CRUST 235 



like sign of deflection residuals are more sharply bounded and 

 smaller in size than the areas of like sign of gravity anomalies. 

 The latter occur commonly in areas so broad that a vertically bal- 

 anced irregularity in the distribution of density would have but 

 little effect. Yet the large gravity anomalies occur in the midst of 

 such large areas, as shown on Fig. 5. There are, furthermore, few 

 sharp reversals of sign of the gravity anomaHes save those at 

 different elevations in mountainous regions and these are explained 

 by the presence of regional compensation. There are, on the 

 contrary, many sharp reversals of the deflection residuals. 



It is to be concluded, therefore, that, although some degree of 

 balancing of irregularities in the same column no doubt exists, 

 this is not a common or controlling explanation of the anomalies 

 and residuals. They are overshadowed by a distribution which 

 points, on the contrary, to regional departures from isostasy by 

 regional excesses or defects in density. 



In the location of stations, the deflection observations are 

 arranged at relatively close intervals and in linear zones, owing to 

 the necessity of triangulation. They give the most information 

 as to the size of areas of relative excess and defect. But two areas 

 of relative excess and defect may both be in absolute excess or 

 absolute defect. The gravity stations are more widely scattered. 

 The local variations are in consequence poorly defined, but the 

 limits of absolute excess and defect of mass are determined with 

 more accuracy. They appear to show that areas as large as 1,000 

 by 2,000 km., 620 by 1,240 miles, may depart in one direction from 

 isostasy, but only to a moderate amount. It is seen from Fig. 5 

 that between Florida and a line drawn from Lake Superior to the 

 Rio Grande the broad areas of less than mean anomaly are negative. 

 From this line a great positive area extends to the northwest. The 

 quarter of the United States bordering the Pacific Ocean is, how- 

 ever, another great region of negative anomalies. Upon these 

 broad regions of mean anomaly or less are superposed smaller and 

 better-defined areas of more than mean anomaly, negative and posi- 

 tive areas occurring in the same broad region. These smaller areas 

 are inclosed by the 0.020 anomaly contour. They commonly 

 range from 300 to 400 km. across, 200 to 250 miles, but the maxima 



