THE FIGURE OF THE EARTH 19 



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, Lon- 

 don, 1901), Chapters 1-2. 



The departure of the spheroid toward the tetrahedron : 



W. LOWTHIAN GREEN. Vestiges of the Molten Globe, Parti. 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 argu- 

 ments for a tetrahedral form). 



W. PRINZ. L'echelle reduite des experiences geologiques, Bull. Soc. Beige 

 d'Astronomie, 1899. 



B. K. EMERSON. The Tetrahedral Earth and Zone of the Interconti- 

 nental Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pis. 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. Engel- 

 mann, Leipzig, 1907. (Contains a valuable series of map plates, 

 showing the probable boundaries of the continents in the different 

 geological periods). 



