4 THE SEAS 



Until quite recently it was thought that after a depth of 

 1,000 fathoms was reached the slope of the sea floor became 

 almost imperceptible and was in most places not unlike a 

 vast and slightly undulating plain. That although it 

 stretched down hundreds of fathoms deeper, its extent was 

 so great that in most places we should be unable to ap- 

 preciate any gradient whatever. From about 2 ,000 fathoms 

 downwards this very gradually shelving ocean bed was 

 known as the Abyss or Abyssal Plain. 



It had long been known that the whole bed of the ocean 

 was not absolutely fiat but presents considerable variation 

 in depth over large areas. There is, for instance, stretching 

 north and south through the whole Atlantic ocean a con- 

 tinuous 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, Ascen- 

 sion and Tristan d'Acunha. 



But the introduction of echo sounding has made it 

 possible for us to obtain many observations for every single 

 sounding made with a lead. As a result our conception 

 of the ocean bed will become materially altered. Although 

 over large areas the vast masses of silt and bottom deposits 

 may tend to smooth out the surface, it is now known that 

 in some regions there are mountain ranges, peaks and cliffs 

 just as we find on land. Most striking, for instance, is the 

 discover}'' of canyons 4,000 feet deep off New York. 



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 



