=The floors of the hydrosphere and atmosphere.=—The highest altitudes upon the continents and the profoundest deeps of the ocean are each removed about 30,000 feet, or nearly 6 miles, from the level of the sea. In comparison with the entire surface of the lithosphere, these extremes of elevation represent such small areas as to be almost inappreciable. Only about 1/80 of the lithosphere surface rises more than 6000 feet above sea level, and about the same proportion lies deeper than 18,000 feet below the same datum plane (Fig. 8). Almost the entire area of the lithosphere is included either in the so-called continental plateau or platform, in the oceanic platform, or in the slope which separates the two. The continental platform includes the continental shelf above referred to, and represents about one third of the entire area of the planet. This platform has a range of elevation from 6000 feet above to 600 feet below sea level and has an average altitude of about 2300 feet. The oceanic platform slopes more steeply, ranges in depth from 12,000 to 18,000 feet below sea level, and comprises about one half the lithosphere surface. The remaining portion of the surface, something less than one eighth of all, is included in the steep slopes between the two platforms, between 600 and 12,000 feet below sea. The two platforms and the slope between them must not, however, be thought of as continuous features upon the surface, but merely as representing the average elevations of portions of the lithosphere. READING REFERENCES FOR CHAPTER II On the evolution of ideas concerning the earth’s figure:— SUESS. The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter 1. V. ZITTEL. History of Geology and Paleontology (Walter Scott, London, 1901), Chapters 1-2. The departure of the spheroid toward the tetrahedron:— W. LOWTHIAN GREEN. Vestiges of the Molten Globe, Part 1. London, 1875. (Now a rare work, but it contains the original statement of the idea.) J. W. GREGORY. The Plan of the Earth and Its Causes, Geogr. Jour., vol. 13, 1899, pp. 225-251 (the best general statement of the arguments for a tetrahedral form). W. PRINZ. L’échelle reduite des expériences géologiques, Bull. Soc. Belge d’Astronomie, 1899. 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