LIFE IN THE DEEP SEA. 577 



small proportion of the elevated masses projects above water, 

 hence from an ordinary map we gain no truer impression of the 

 form of the sculpturing of the surface of the earth itself than we 

 should of the shape of a range of mountains if we viewed it when 

 all but its summits were hidden by a flood. 



So small a proportion does the mass of dry land elevated 

 above sea-level bear to the hollows on the earth's surface beneath 

 this level, that the cavities now occupied by the sea would con- 

 tain three times the volume of the earth existing above the sea 

 surface. If the surface of the land and the sea bottom were 

 brought to a complete level, the waters of the sea covering its 

 even face would still have a depth of about 1,700 fathoms, being 

 reduced in depth by the process only about 800 fathoms * 



We should obtain a more correct idea of what are the real 

 elevations and what the depressions on the earth's surface, if we 

 drew on the map or globe a contour line marking the level at 

 which the mass of the earth raised above this line is just equal 

 to the excavations beneath it, and would just fill up these hollows 

 if the surface of the earth were rendered even and smooth. 



Although the depth of the ocean is so small in proportion to 

 the vastness of its expanse, the depth is, nevertheless, so great 

 as to be difficult of adequate realisation. The greatest depth as 

 yet ascertained by sounding occurs, as will be seen from the map 

 at the commencement of this work, in the North West Pacific 

 Ocean ; it amounts to about five miles and a quarter. 



In order to realize such a depth, the reader should think of a 

 spot distant several miles from his actual position, and then 

 attempt to project the distant point downwards, until it lies 

 vertically beneath him. The average depth of the ocean between 

 lats. 60° K and 60° S.f is about three miles or 2,500 fathoms. 

 The great depth of five miles occurs only exceptionally over 

 very small areas. 



The vastness of the depth of the Ocean was constantly 

 brought home to us on board the " Challenger " by the tedious 



* O. Peschel, " Neue Probleme der Vergleichenden Erdkunde." Leipzig, 

 1876, s. 82. 



t J. J. Wild, " Thalassa," pp. 14-15. London, Marens Ward & Co., 

 1877. 



P P 



