276 A CENTURY OF GEOLOGY. 



increasing weight of accumulating sediments/ It was afterwards 

 applied to continental masses by Archbishop Pratt ^ and by the roA^al 

 astronomer, Professor Air3^^ But for its wide application as a prin- 

 ciple in geology, its clear definition, and its embodiment in an appro- 

 priate name we are indebted to Major Dutton, United States Army.* 



The principle may be briefly stated as follows: A globe so large as 

 the earth, under the influence of its own gravity, must behave like a 

 ver3^ stifily viscous body — that is, the general form of the earth and 

 its greatest inequalities must be in substantial static equilibrium. For 

 example, the general form of the earth is oblate spheroid, because that 

 is the only form of equilibrium of a rotating body. Rotation deter- 

 mines a distribution of gravity with latitude which brings about this 

 form. With any other form the earth would be in a state of strain 

 to which it must slowly yield and Anally relieve itself by becoming 

 oblate. If the rotation stopped, the earth would accommodate itself 

 to the new distribution of gravity and become spherical. 



The same is true of the large inequalities of surface. Oceanic 

 basins and continental arches must be in static equilibrium or they 

 could not sustain themselves. In order to be in equilibrium the sub- 

 oceanic material must be as much more dense than the continental and 

 subcontinental material as the ocean bottoms are lower than the con- 

 tinental surfaces. Such static equilibrium, by difl'erence of densit}^, 

 is completeh" explained by the mode of formation of oceanic basins 

 already given. 



So also plateaus and great mountain ranges are at least partly sus- 

 tained by gravitative equilibrium, but partly also b}' earth rigidit}'. 

 It is onl}' the smaller inequalities, such as ridges, peaks, valleys, etc., 

 that are sustained by earth rigidity alone. 



These conclusions are not reached b}^ physical reasonings alone, but 

 are also confirmed by experimental investigations. For example, a 

 plumb line on the plains of India is deflected indeed toward the Him- 

 alayas, as it ought to be, but much less than it would be if the moun- 

 tain and submountain mass were not less dense and the suboceanic 

 material more dense than the average.^ Again, gravitative determi- 

 nations by pendulum oscillations, undertaken by the United States 

 along a line from the Atlantic shore to Salt Lake City, show that the 

 largest inequalities, such as the Appalachian bulge, the Mississippi 

 basin hf)llow, and the Kock}^ Mountain bulge, are in gravitative equi- 

 librium — i. e., the mountain and submountain material is as much 



1 Philosophical Magazine, Vol. II, p. 212, 1837; Quarterly Journal of Geological 

 Society, Vol. II, p. 548, 1837. 



''Philosophical Magazine, Vol. IX, p. 231, and Vol. X, p. 240, 1855. 



^ Philosophical Transactions, 1855, p. 101. 



* Philosophical Society of Washington, 1892. 



■^ Pratt, Philosophical Magazine, Vol. IX, p. 231, 1855; Vol. X, p. 340, 1855; Vol. 

 XVI, p. 401, 1858. 



