294 Dwellers of the Sea and Shore 



main cause regulating this is temperature, which in all 

 seas is the same after the depth of a little more than 

 a mile. 



The deepest place yet discovered in the ocean is the 

 Philippine Deep which has a depth of six miles. Some 

 conception of this terrific depth may be obtained when 

 I say that it could contain Mount Everest, the highest 

 mountain in the world, and still submerge it with more 

 than a mile of water over the top. The term "deep" 

 is one applied by marine naturalists to those parts of 

 the ocean that are shown by soundings to be 18,000 

 feet or deeper. There are known to be about sixty of 

 these deeps. Depths of more than 24,000 feet have 

 been recorded in ten of them, two of these abysses 

 being in the Atlantic and the remainder in the Pacific. 

 Between the dry land and the various deeps exists a 

 well-defined area known as the continental shelf, a com- 

 paratively shallow region, constantly swept by tides, 

 waves, and oceanic currents to a depth of 600 feet; 

 and it is separated from the deeps by an abrupt 

 declivity, where the depth of the water increases 

 rapidly, called the continental slope. No doubt many 

 portions of the continental shelf were, at no very re- 

 mote period, regions of dry land; for crossing it in 

 numerous places are deep ravines that are believed to 

 be submerged river valleys. In some places their con- 

 nection with still existing rivers can be traced. 



In the abyssal realm, wave action no longer makes 

 itself felt, the oceanic currents are absent, and the water 

 is still and cold — almost freezing — and utterly without 

 any light from the sun. The darkness is relieved only 

 by the light of phosphorescent animals. The enormous 



