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SCIENCE 



[N. S. Vol. XLI. No. 1048 



of radioactive origin. More Information is 

 certainly in store for us, for Mr. Michelson 

 is now measuring the terrestrial tides in 

 terms of the wave-length of light, while 

 methods have been developed by which the 

 distribution of density above the level of 

 isostatic compensation can be studied. 



Thus the future is full of hope. The 

 rational method of attaining it is to make 

 trial hypotheses, and to devise methods of 

 testing them. 



Laplace seems to have been the first to 

 grasp the problem of isostasy and in 1818 

 he maintained that the irregularities of 

 the earth and the causes which disturb its 

 surface extend to but a small depth. I do 

 not find in his memoirs any rigorous proof 

 of this interesting anticipation. Sir John 

 Herschel in 1833 regarded the earth as a 

 yielding mass and considered erosion and 

 deposition as the primum mobile of geology, 

 but he did not pursue the subject. Arch- 

 deacon Pratt in 1858 first expressed the 

 hypothesis of isostasy in exact terms, dis- 

 cussing it mathematically and adducing 

 evidence in its favor from the geodetic 

 survey of India. 



As we all know, the enormous labor need- 

 ful to prove the hypothesis from deflections 

 of the vertical was undertaken by Mr. John 

 F. Hayford. Mr. Helmert characterized 

 Hayford's investigation as "truly magni- 

 ficent" and called the underlying idea the 

 Pratt-Hayford hypothesis. 



Helmert himself devised a method of 

 testing isostasy by observations on the in- 

 tensity of gravity instead of deflections. 

 His results coincide almost exactly with 

 Hayford's and thus immensely strengthen 

 the theory. At Helmert 's suggestion also 

 Mr. O. Hecker made many observations on 

 the intensity of gravity at sea. These ob- 

 servations are indeed of inferior accuracy, 

 but suffice to prove that isostatic compensa- 

 tion exists beneath the Atlantic and the 



Pacific as well as under the United States. 



In my opinion the geodetic evidence for 

 isostasy is so manifold and so consistent as 

 to amount to proof. Equilibrium is nearly 

 or quite complete at a depth of between 110 

 and 140 kilometers, the most probable 

 value being near 120 kilometers. 



Messrs. Hayford and Bowie have also 

 investigated the effect of isostasy on the 

 intensity of gravity in the United States 

 and at selected stations in other parts of 

 the world. This research confirms the 

 existence of isostasy, but reveals certain 

 anomalies due either to imperfect compen- 

 sation some 120 kilometers from the sur- 

 face or to irregularities in the distribution 

 of density, or to both causes. 



It is shown in my paper that the largest 

 deflections in the United States and also 

 the largest anomalies could be accounted 

 for by a spherical batholith, say of peri- 

 dotite, just buried beneath the surface and 

 having a diameter of 8^ miles. Of course 

 such a batholith would not be considered 

 surprising by geologists. When the exist- 

 ence and abundance of dikes, sills and lac- 

 coliths at all accessible levels is considered, 

 as well as the probability of their prevalence 

 at aU levels above the deepest volcanic foci, 

 it appears that heterogeneities in the earth's 

 outer shell are of the order of magnitude 

 needful to account for the gravity anom- 

 alies. In short there is much evidence for 

 the conclusion that compensation at the 

 compensation level is very nearly complete. 



If so, the mass beneath that level must be 

 almost free from strain and can have cooled 

 but little from its primeval temperature. 



On the isostatic theory the continents 

 stand out above the bottom of the ocean be- 

 cause of inferior density. This inferiority 

 may be due to higher temperature or to 

 voids such as joints, or to both. A third 

 possibility is that it might be due to litho- 

 logical differences, but of that there is no 



