164 



CONTJNEXTAL PROBLEMS OF GEOLOGY. 



Differentiation of continentid and oeeuniv plateaus. — It is one of the 

 paradoxes of the subject that our ideas as to the essential character of 

 the continents liave been greatly modified and claritied by the recent 

 exploration of the seai. Tlie work, especially, of the Challenger and 

 the BlaJce in delineating' and sampling- the bottom of the ocean has 

 given new dctinitions, not only to the term ''deep sea, "but also to the 

 term "continent," as they are employed by students of terrestrial 

 mechanics and of ijhysical geography. To the continental lands are 

 now added the continental shoals, and the depth of the deep sea is no 

 longer its sole characteristic. Look for a moment at this generalized 

 profile of the earth's surface. It expresses in a concise way the rela- 

 tions of area to altitude and of both to the level of the sea. Murray, to 

 whose generalizations from the Challenf/er dredgings and soundings 

 the student of continents owes so much, has computed, with the aid 

 of the great body of modern data, the areas of land and ocean bed con- 

 tained between certain contours, fourteen in number,* and from his 



CONT! NLNTAL 

 PLATEAU 



♦ 30.000 FT., 



♦20,000 rr. 



• 10.000 FT. 

 SEA LEVEL 

 - i OtOOO FT. 

 feSOdOOO FT. 

 -5^0.000 FT. 



Figure Ij—Oencralued Profile, showing relative Are.as of the Earth's Surface at different Heights and 



Depths. 



figures I have constructed the iHotile. Vertical distances represent 

 heights and horizontal distances represent terrestrial areas. The full 

 width of the diagram from side to side stands for tlie entire surface of 

 the earth. The striking features of the profile are its two terraces or 

 horizontal elements. Two-fifths of the earth's area lies between 11,000 

 and 10,000 feet beneath the ocean, constituting avast submerged pla- 

 teau, whose mean altitude is — 14,000 feet. This is the plateau of the 

 deep sea. One-fourth of the earth's area falls between the contour 

 5,000 feet above the ocean and the contour 1,000 feet below, and has a 

 mean altitude of + 1,000 feet. This is the continental plateau. The two 

 plateaus together comprise two-thirds of the earth's surface, the re- 

 maining third including the intermediate slopes, the areas of extreme 

 and exceptional depth, and the areas of extreme and exceptional height. 



* John Murray: "On tho hei.alii^ of the laud and tlie depth of the ocean." Scottish 

 Geograiiliical Mag., vol. iv, 1888, p. I. 



