30 JOSEPH BARRELL 



are to be regarded in most cases as situated to one side of the stations 

 and somewhat greater. But exceptional and local maxima must 

 be smoothed out to form a part of the harmonic curve. It is, 

 furthermore, the difference of anomaly between adjacent opposite 

 phases which is the significant feature. This difference runs from 

 0.060 to 0.080. The latter figure will be chosen. For this wave- 

 length, representing a certain unit area of attraction, the best 

 divisor is perhaps to take 0.0024 dyne of anomaly as equivalent to 

 100 feet of rock. An anomaly of o . 080 dyne is on that basis equiva- 

 lent to 3,330 feet (1,015 ^0 of rock. The crust of the United 

 States sustains, therefore, harmonic loads 600 km. (373 miles) in 

 wave-length and 1,015 m. (3,330 feet) in total amplitude. The 

 stresses which this wave-series imposes on the crust are shown by 

 curve B, Fig.. 18. 



Departures from isostasy of large wave-lengths. — For the continent 

 as a whole and in its relations to the ocean basins isostasy is nearly 

 perfect; but the question rises here, how nearly? The first term 

 of the gravity formula for the Vienna system of gravity observations 

 is 978.046 dynes. The first term for the Potsdam system is 

 978.030. The first term for the United States system after reject- 

 ing the Seattle anomalies is, as shown by Bowie, 978.038 dynes. 

 These respective systems differ as a whole by these amounts. We 

 have no right to assume that any one is absolutely correct. The 

 whole of the United States system may lie a little above or below 

 the level giving isostatic compensation with respect to the average 

 surrounding ocean basins, or with respect to the entire earth. The 

 mean value for the United States suggests, however, that, as a 

 whole, the continent lies within a few hundred feet, possibly less 

 than one hundred feet, of the level which would give perfect 

 isostatic equilibrium. 



Let us consider next its larger parts. These can be compared 

 with each other and with the United States as a whole. Although, 

 as discussed in Part IV, the map of gravity anomahes lacks detail, 

 the grouping of many stations of like sign into large areas gives 

 confidence in the conclusion that there are regional departures 

 from isostasy. These are of two or three orders of magnitude, of 

 which the areally smaller have been discussed. To bring out the 



