xi The Bed of tJie Oceans 191 



about 2 200 feet above sea-level, while the sea covering 

 the remaining 141,000,000 square miles of surface has 

 an average depth of 12,600 feet, or 2100 fathoms ( 355). 

 The loftiest point of the land, Mount Everest in the 

 Himalayas, reaches to 29,000 feet above sea-level, and 

 the deej^st parts of the Pacific Ocean descend to a depth 

 of Y2. 8, 200 feet below sea-level. The whole vertical range 

 on the surface of the lithosphere is thus about 60,000 feet, 

 nearly 12 miles, which is only -^^ of the Earth's diameter. 

 The narrow crest of the Elevated Region forming the 

 visible land has only ^ of the volume of the ocean hollows, 

 and thus the average level of the solid Earth evidently lies 

 beneath the sea-surface, and the summits of the land rise 

 higher above the mean level than the depressions of the 

 ocean sink below it. 



254. Mean Sphere Level. From Murray's figures, the 

 position of the mean surface of the lithosphere (mean sphere 

 level) was calculated by the author to be about 10,000 feet 

 (1700 fathoms) below the present sea-level, or more than 

 half-way down the slope which separates the two great 

 regions. If we imagine a transparent shell, similar in 

 form to the Earth and concentric with it, to cut this slope 

 at the level indicated, the volume of all the elevations 

 projecting above the shell would be precisely equal to the 

 volume of all the depressions extending below it. By a 

 remarkable coincidence, one-half of the area of the Earth's 

 surface is above mean sphere level and one -half below. 

 The line of mean sphere level traced on a map (PI. XIV.) 

 thus serves to divide the surface of the ^lithosphere into a 

 depressed and an elevated half. 2 



255. Three Areas of the Lithosphere. The depressed 

 half of the lithosphere is called by Dr. Murray the Abysmal 

 Area, all parts of which are always covered by water more 

 than 10,000 feet deep. The upper part of the elevated 

 half of the lithosphere forms the Continental Area, which 

 is always above water, and occupies rather more than one- 

 quarter (28 per cent) of the surface. The remainder of 

 the surface, measuring somewhat less than one-quarter 

 (22 per cent), and always covered by water less than 



