442 POPULAR SCIENCE MONTHLY. 



condition of tlie evolution of the eartli, these wide oscillations, by 

 increasing and decreasing the size and height of continents and 

 changing greatly their contours, have determined all the details of 

 the drama enacted on the surface, and were the determining cause 

 of the varying rates and directions of the evolution of the organic 

 kingdom. These were the cause of the unconformities and the cor- 

 responding apparent wholesale changes in species so common in the 

 rocky strata, and which gave rise to the doctrine of catastrophism 

 of the early geologists. These also have so greatly modified the 

 contours of the continents and their size by temporary increase or 

 decrease that they have obscured the general law of the steady de- 

 velopment of these, and therefore their substantial permanency. 



Although the most important of all crust movements in deter- 

 mining the whole history of the earth, and especially of the organic 

 kingdom, we shall dwell no further on them, because no progress 

 has yet been made in their explanation. This, too, must be left 

 to the workers of the twentieth century. 



[llie Frinciple of I sostasy.— The principle of static equilibrium 

 as applied to earth forms was first brought forward (as so many 

 other valuable suggestions and anticipations in many departments 

 of science) by the wonderfully fertile mind of Sir John Herschel, 

 and used by him in the explanation of the sinking of river deltas 

 under the increasing weight of accumulating sediments.* It was 

 afterward applied to continental masses by Archbishop Pratt f and 

 by the Iloyal Astronomer Professor Airy.:}: But for its wide appli- 

 cation as a principle in geology, its clear definition, and its embodi- 

 ment in an appropriate 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 very stiffly viscous body — that is, the general form of the 

 earth and its greatest inequalities must be in substantial static 

 equilibrium. Por example, the general form of the earth is oblate 

 spheroid, because that is the only form of equilibrium of a rotating 

 body. Rotation determines a distribution of gravity with latitude 

 which brings about this form. AVith any other form the earth 

 would be in a state of strain to which it must slowly yield, and 

 finally relieve itself by l)ecoming oblate. If the rotation stopped, 

 the earth would accommodate itself to the new distribution of 

 gravity and become spherical. 



* Philosophical Magazine, vol. ii, p. 212, 1837; Quarterly Journal of Geological Society, 



vol. ii, p. 548, 1837. 



t Pliilosophifal Magazine, vol. ix, p. 2:?1, and vol. x, p. 240, 1865. 



+ riiilosupliiual Trans., 1855, p. 101. * Philosophical Society of Washington, 1892. 



