320 Dzcellers of the Sea and Shore 



almost exactly 2,300 feet. The sea, on the other hand, 

 has a mean depth of more than two miles. Nine tenths 

 of it is more than a mile deep, and two thirds of its 

 depth is more than two miles. 



The ocean basin, on the whole, is very smooth and 

 stretches out in great level plains. All the greatest 

 depths seem to occur in trenches, rather than in gradu- 

 ally sloping valleys. Most of them are narrow, but are 

 quite long, and their steeper side is supposed to be the 

 result of a fracture in the crust of the earth. This is 

 indicated by the fact that deeps are more often asso- 

 ciated with regions of frequent submarine earthquakes. 

 But the great deeps, those of excessive depths, are not 

 nearly so extensive as is commonly imagined; the total 

 area where the depth is more than three miles is prob- 

 ably not more than one one hundred and forty-fifth of 

 that of the sea floor. Life has been found in all of 

 them, but at those depths any object is under a pressure 

 of five tons to the square inch. 



It is in the Atlantic Ocean that the newer and trust- 

 worthy methods of sounding have chiefly been em- 

 ployed, and the contours of its floor may now be con- 

 sidered as fairly well known. The greatest depth is in 

 the Virgin Islands Deep, which w^as measured by the 

 Challenger, and it lacks 510 feet of being four and a 

 half miles deep. But over the greater part of the 

 Atlantic basin, the depth ranges between two and one- 

 half and three and one-half miles. Along the middle, 

 in a north-and-south direction, runs a great irregular 

 ridge on which the depth is less than two miles, and a 

 plateau of about the same depth extends almost con- 

 tinuously from Newfoundland to the British Isles. It 



