THE STRENGTH OF THE EARTH'S CRUST 29 



Seattle and Olympia are 80 km. apart and the difference of 

 their anomahes is o . 1 26 dyne. If the anomaHes had been measured 

 at the points giving maximum values they would certainly give a 

 difference of at least o. 130 or perhaps o. 140, about five times the 

 amplitude of the variations between Washington and Hoboken. 

 This large value seems, however, to be exceptional for the United 

 States and may constitute but a single wave. We may take it, 

 however, as showing that the crust can sustain a harmonic load of 

 160 km. (100 miles) wave-length and total ampHtude of 0.120 

 dyne. For the reasons explained in Part IV, especially on p. 304, 

 the divisor to be used to turn this anomaly into the equivalence 

 of rock measured in feet could not be over 0.0018 and a better 

 figure for the interpretation of this short wave-length is 0.0015. 

 This gives an amplitude of 8,000 feet (2,440 m.). The part below 

 the mean level is then 50 miles wide and 4,000 feet deep, the 

 adjacent positive parts of the wave being of equal dimensions but 

 in the opposite direction. This is of the same order of relief as 

 the larger ranges of folded mountains and intermontane valleys. 

 The stresses which this harmonic series imposes upon the crust are 

 shown by curve A of Fig. 18. 



Maximum loads for mean wave-lengths. — In Part II it was argued 

 that the evidence of anomalies from mountain stations showed 

 regional compensation on the average probably to the outer limit 

 of zone O, radius 166.7 km., diameter consequently 333.4km. 



Over the United States in general the intercepts of the areas 

 of grouped deflections averaged 180 miles. The average diameter 

 would therefore doubtless be as great as 200 miles (320 km.). 



On Bowie's map of the New Method gravity anomalies,^ it is 

 seen that the distances from pronounced maxima to pronounced 

 minima average 250-350 km. 



From these several lines of evidence we may conclude with some 

 confidence that the half wave-length for pronounced anomaHes in 

 the United States averages near 300-350 km. The wave-length 

 is therefore 600-700 km. (373-435 miles). A wave-length of 

 600 km. will be taken. The pronounced maxima for these waves 

 runs from plus or minus 0.030 to 0.060 dyne. The real maxima 



^ This article, Part II, Jour. Geol., XXII (191 4), 153. 



