PRELIMINARY OUTLINE 



Per cent 



More than 6,000 feet above sea-level 2.3 



Between sea-level and 6,000 feet above 25 . 5 



Between sea-level and 6,000 feet below 14 .8 



Between 6,000 and 12,000 feet below sea-level 14.8 



Between 12,000 and 18,000 feet below sea-level 39.4 



Between 18,000 feet and 24,000 feet 3.1 



From these estimates it appears that if the surface were graded 

 to a common level by cutting away the continental platforms and 

 dumping the matter in the ocean basins, the average plane would 

 lie somewhere near 9,000 feet below sea-level. The continental 

 platform may be conceived as rising from this common plane 

 rather than from the sea-level. 



Epicontinental seas. Those shallow portions of the sea which 

 lie upon the continental shelf, or extend into the interior of the 

 continent, such as the Baltic Sea and Hudson Bay, may be called 

 epicontinental seas, for they really lie upon the lower border of the 

 continental platforms. Those detached bodies of water which 

 occupy deep depressions in the surface are to be regarded as true 

 abysmal seas. Such, for example, are the Mediterranean and 

 Caribbean seas and the Gulf of Mexico, w T hose bottoms are as 

 low as many parts of the true ocean basin itself. 



Diversities of surface. The bottoms of the oceanic basins are 

 diversified by broad undulations which range through many thou- 

 sands of feet, but they have not those irregularities of form that 

 give variety to land surfaces. The ocean bottoms are also diversi- 

 fied by volcanic peaks, many of which rise to the surface and con- 

 stitute isolated islands. From many of them, the solid surface 

 slopes rapidly down to abysmal depths, so that many of the volcanic 



