318 H. F. REID ISOSTACY AND EARTH MOVEMENTS 



equilibrium. There is some other cause, which we do not know, that pro- 

 duces these great movements. 



But the principle of isostasy is of great value. It bears some analogy 

 to the principle of the conservation of energy in the physical sciences. 

 It does not always tell us what earth movements will take place, but it 

 tells us that these movements must be of such a character that the amount 

 of matter underlying a given area of the earth's surface is never ma- 

 terially increased or decreased ; and if there be a transfer of matter at the 

 surface by erosion and deposition, by folding of the strata or by any other 

 means, then there must be a corresponding subterranean transfer in the 

 opposite direction. 



Isostasy could not be proved by geologic observations, although they 

 might suggest it. Its proof is a geodetic proof, depending entirely on the 

 deflection of the vertical and the anomalies of gravity, for it is only by 

 such determinations that the masses underlying different regions can be 

 compared. 



Evidences of isostatic Equilibrium 



The existence of isostatic equilibrium over the area of the United 

 States has been proved by the determinations of Hayford and Bowie. 4 

 These observations were made in regions of erosion and regions of depo- 

 sition, at high and low altitudes, near the coast and in the interior, in 

 regions where earth movements are active and in regions that have long 

 been stable. The conclusions have been corroborated by the work of the 

 Great Trigonometrical Survey of India, with one station at an altitude 

 of 15,427 feet, in Tibet. 5 The principle has also been applied to stations 

 situated on islands, and to a number of stations in Europe, some in the 

 Alps, and with satisfactory results. Nansen swung a pendulum on the 

 ice, and in the Fram, over the Arctic Ocean, and found gravity normal. 6 

 Hecker made determinations over the Atlantic, the Pacific, and the In- 

 dian oceans, and found only small deviations from the normal in the 

 force of gravity. 7 We are therefore forced to the conclusion that, at the 



4 John F. Hayford : The figure of the earth and isostasy. U. S. Coast and Geodetic 

 Survey, Washington, 1900. Supplementary investigation of the figure of the earth and 

 isostasy. Same, 1910. 



John F. Hayford and William Bowie : The effect of topography and isostatic com- 

 pensation upon the intensity of gravity. Special Publication No. 10. Same, 1912. 



William Bowie : Second paper. Special Publication No. 12. Same, 1912. Investiga- 

 tions of gravity and isostasy. Special Publication No. 40. Same, 1917. 



5 Col. Sir Sidney Burrard : A brief review of the evidence upon which the theory of 

 isostasy has been based. Geog. Jour., 1920, vol. lvi, pp. 47-59. 



6 The Norwegian North Polar Expedition of 189.^-1896, vol. ii, pt. viii. Pendulum ob- 

 servations by O. E. Schiotz. 



7 Veroff. d.Kon. Preuss. Geodatischen Institutes, Nos. 11 (1903), 10 (1908), 20 (1910). 



