Page:Popular Science Monthly Volume 56.djvu/456

442 condition of the evolution of the earth, 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 corresponding 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 development of these, and therefore their substantial permanency.

Although the most important of all crust movements in determining 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.

The Principle of Isostasy.—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 and by the Royal Astronomer Professor Airy. But for its wide application as a principle in geology, its clear definition, and its embodiment 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. For 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. With any other form the earth would be in a state of strain to which it must slowly yield, and finally relieve itself by becoming oblate. If the rotation stopped, the earth would accommodate itself to the new distribution of gravity and become spherical.