THE STRENGTH OF THE EARTH'S CRUST 37 



of the uncompensated portion of the crust-waves be measured in 

 terms of anomaly by taking the algebraic sum of the anomalies 

 over the plateau and the deep, a total amplitude is obtained of 

 0.374 or a half-ampUtude of 0.187. On the island of Hawaii 

 an observation on Mauna Kea at an elevation of 3,981 m. gave a 

 New Method anomaly of +0.183, almost the same figure as the 

 half-ampHtude for the great Tonga crust-wave. 



Helmert has discussed the gravity disturbances found in the 

 Hawaiian Islands and states of them: "For the Hawaiian Islands 

 it must be concluded on the whole that a part of the mass gives 

 rise to positive gravity disturbances and only the remainder is 

 isostatically supported. If the disturbances were produced solely 

 by the mass of the islands the values of Ag and Ag" [the disturb- 

 ances of gravity] would be somewhat greater than they are found."' 



From this review of the mountain chains of the Pacific it may be 

 concluded that the ocean floor can sustain a harmonic wave-length 

 of 400 km. which gives an anomaly at the crest lines as great as 

 that observed on Mauna Kea, 0.183 dyne. To interpret this as 

 an equivalent load of rock a divisor must be selected. The divisor 

 depends upon the depth and distribution of compensation and the 

 area of the region of outstanding mass. As shown in Part IV, 

 p. 311, 0.0024 dyne might reasonably be chosen as the amount of 

 anomaly equivalent to 100 feet of rock, but for these great loads it 

 is desirable to lean toward the sid-e of an underestimate. Therefore 

 0.0030 will be taken as the divisor. This gives 1,868 m. as the 

 crest height of the uncompensated portion of the Hawaiian moun- 

 tain chain. The same applies to the Tonga fold. If, however, 

 0.0024 should be chosen as the divisor, then o. 183 dyne of anomaly 

 would correspond to a half -amplitude of 2,334 m. 



It may be taken then as fairly certain that these great moun- 

 tain chains show reliefs which depart as much as 2,000 m. above and 

 below the mean level which would give perfect isostasy. It may 

 be concluded in consequence that the oceanic crust can sustain 

 a harmonic wave-length of 400 km. with an uncompensated 

 ampHtude measured by 4,000 m. of rock. The diagram of 



■ "Die Schwerkraft und die Massenverteilung der Erde," Ency. math. Wissen- 

 schaften, Band VI, i, B, Heft 2, (1910), p. 133. 



