CHAPTER VI 

 THE OCEAN AND ITS WORK 



The oceans and seas cover about 72 per cent, of the surface of the 

 earth. The average depth of the oceans is about two and one half 

 miles, that of the Pacific being somewhat greater than that of the 

 Atlantic ; the average heigh.t of the continents, however, is only a 

 little more than 2000 feet. If all the dry land above sea level were 

 washed into the sea, it would fill only one fortieth of that depression. 

 Soundings to a depth of 32,088 feet have been made in the Pacific 

 Ocean near Mindanao, P. I., and to a depth of nearly 28,000 feet 

 near Japan (Tuscarora Deep). Within 70 miles of Porto Rico the 

 ocean bottom descends to 27,366 feet, and within 10 miles of the 

 Bermuda Islands depths of 17,460 feet are encountered. These 

 great depths are not of wide extent, but are almost as limited as are 

 the great heights of the continents. Moreover, the greatest depth 

 of the oceans is practically the same as the greatest mountain height, 

 each being about six miles. There are, however, few such excessive 

 differences in elevation in short distances on the land as there are 

 differences in depth in the ocean, although Mt. Everest (29,002 feet) 

 is within 60 miles of the nearly sea level Ganges plain, and the vol- 

 cano Fuji in Japan rises 12,400 feet directly from sea level. 



General Character of the Ocean 



Topography of the Ocean Floor. — In order to gain a true concep- 

 tion of the topography of the ocean bottom, it must be borne in mind 



(1) that stream erosion, which is continually at work on the land and 

 which tends to roughen its surface, is absent on the ocean bottom; 



(2) that minor depressions which may exist temporarily tend to be 

 rapidly filled by the sediments brought to the ocean from the land 

 and by the material carried in solution, some of which is precipitated 

 directly and some absorbed by animals to form their shells and skele- 

 tons, only to be left upon the ocean floor after their death. 



Bordering practically all of the shore lines of the oceans is a belt of 

 water which has a depth of less than 600 feet and is from several miles 



194 



