B. K. Emerson. The Tetrahedral Earth and Zone of the Intercontinental Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14.
M. P. Rudski. Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters 1-3 (the best discussion of the geoid from the purely mathematical standpoint, so far as the spheroid is concerned).
The earlier figures of the earth:—
Th. Arldt. Die Entwicklung der Kontinente und ihrer Lebewelt. Engelmann, Leipzig, 1907. (Contains a valuable series of map plates, showing the probable boundaries of the continents in the different geological periods).
CHAPTER III
THE NATURE OF THE MATERIALS IN THE LITHOSPHERE
The rigid quality of our planet.—For a long time it was supposed that the solid earth constituted a crust only which was floated upon a liquid interior. This notion was clearly an outgrowth of the then generally accepted Laplacian hypothesis of the origin of the universe, which assumed fluid interiors for the planets, the crust being suggested by the winter crust of frozen water upon the surface of our inland lakes. To-day the nebular hypothesis in the Laplacian form is fast giving place to quite different conceptions, in which solid particles, and not gaseous ones, are conceived to have built up the lithosphere. The analogy with frozen water has likewise been abandoned with the discovery that frozen rock, instead of floating, sinks in its molten equivalent.
Yet even more cogent arguments have been brought forward to show that whatever may be the state of aggregation within the earth’s core—and it may be different from any now known to us—it nevertheless has many of the properties recognized as belonging to solid and rigid bodies. Provisionally, therefore, we may regard the earth’s core as rigid and essentially solid. It was long ago pointed out by the late Lord Kelvin that if our lithosphere were not more rigid than a ball of glass of the same size, it would be constantly passing through periodic six-hourly distortions of great amplitude in response to the varying attractions of the moon. An equally striking argument emanating from the same high authority is furnished by the well-known egg-spinning demonstration. For illustration, Kelvin was accustomed to take two eggs, one boiled and the other raw, and attempt to spin them upon their ends. For the boiled, and essentially solid, egg this is easily accomplished, but internal friction of the liquid contents of the raw egg quickly stops any rotary motion which is imparted to it. Upon the same grounds it is argued that had the earth’s interior possessed the properties of a liquid, rotation must long since have ceased.