230 THE OCEAN 



(1) A great elevated plain, comprising the 

 surface of the continents, estimated to have 

 an average height of about 2250 feet above 

 sea-level and to cover an area of about 57 

 millions of square miles : add to this the 

 continental shelf, extending from the shore- 

 line to the 100-fathoms line and covering an 

 area of about 10 millions of square miles, and 

 we have what may be called the continental 

 area, occupying in all about 67 millions of 

 square miles, or about one-third of the super- 

 ficial area of the globe ; 



(2) a connecting slope, from the 100-fathoms 

 line down to a depth of about 1700 fathoms 

 (the mean sphere level), which is called the 

 continental slope, occupying about 30 millions 

 of square miles, or about one-sixth of the 

 superficial area of the globe ; and 



(3) a great submerged plain — ^the floor of 

 the ocean-basins — estimated to have an aver- 

 age depth of about 15,000 feet below sea-level, 

 which is called the abyssal area, occupying 

 about 100 millions of square miles, or fully 

 one-half of the superficial area of the globe. 



The upper surface of the continental plain 

 stands, then, on an average about two and 

 a half miles above the submerged plain 

 that forms the floor of the ocean. From 

 this great submerged region volcanic cones 

 frequently rise above the surface of the 



