4 THE SEAS 



After a depth of 1,000 fathoms is reached the slope of the 

 sea floor becomes almost imperceptible and is probably in 

 most places not unlike a vast and slightly undulating plain. 

 Although it stretches down hundreds of fathoms deeper, 

 its extent is so great that in most places we should be unable 

 to appreciate any gradient whatever. 



From about 2,000 fathoms downwards this very gradually 

 shelving ocean bed is known as the Abyss or Abyssal Plain. 

 Although almost level compared with our standards on 

 land, the whole bed of the ocean is not absolutely flat but 

 presents considerable variation in depth over large areas. 

 There is, for instance, stretching north and south through 

 the whole Atlantic ocean a continuous ridge between one 

 thousand and two thousand fathoms in depth surrounded 

 on either side by water down to 4,000 fathoms deep (Plate 3). 

 From this ridge rise the oceanic islands of the Azores. The 

 Saint Paul Rocks, Ascension and Tristan d'Acunha. But 

 it is only in the immediate vicinity of these islands that there 

 is any appreciable slope. Soundings have been taken 

 continuously over great distance by transoceanic cable- 

 laying ships, and their results bear out this point : on 

 one occasion only as much as 250 feet being the extreme 

 range of variation in depth through a distance of 100 miles, 

 in water more than 2,500 fathoms deep. Around the 

 before-mentioned oceanic islands which are volcanic in 

 origin, the slopes may, however, be considerable, being as 

 much as 62 for a short distance off Saint Paul. 



The regions in which the bottom lies below a depth of 

 3,000 fathoms are known as " deeps." The greatest depth 

 yet recorded is 5,350 fathoms off the island of Mindanao 

 in the Philippines, in the Pacific. This enormous depth, 

 over six and a quarter miles, is hard to visualize. The 

 reader can perhaps best realize it if he imagine that the 

 highest mountain in the world, Mount Everest, be sunk 



