FOUNDATIONS OF THE GREAT DEEP 9 



need a complete classification of the forms. Those features that 

 are essential to our thinking include, besides the major basins 

 themselves: (i) the continental terrace that flanks each of the 

 larger masses of land; (2) the deep-sea or thalassic island with 

 its great submerged pedestal; (3) the shoal or "bank"; (4) the 

 broad "plateau," rising 1000 fathoms to 1500 fathoms above the 

 general sea floor, though in average still 1000 fathoms or more 

 below sealevel ; (5) the "swell," with the same order of upward 

 projection but arch-shaped and elongated in ground-plan, as 

 illustrated by the Mid-Atlantic Swell (Figure 1); (6) the 

 "deep," a long, relatively narrow trough with water depth 1000 

 to 3000 fathoms greater than the ruling depth of the open 

 ocean. 



After a rapid flight from the land across the continental 

 terrace, we shall range far and wide over deep water, in the 

 effort to find the nature of the substructure supporting the 

 ocean outside the terrace and between the island pedestals. 



Continental Terraces 



Each continent may be regarded as itself a more or less 

 rugged table-land, projecting nearly three miles above the floor 

 of the open ocean. As a rule the surface of the table-land does 

 not fall at a constant rate to this considerable depth. From the 

 shoreline out to distances of 10 to 150 miles the descent is 

 gentle. At the oceanward limit of this nearly flat belt, the 

 water has a depth averaging between 300 and 600 feet; to use a 

 technical term, that outer limit lies between the 300-foot or 

 50-fathom isobath and the 600-foot or 100-fathom isobath. The 

 gently sloping plain is called the continental shelf. See Figure 

 2. From the outer limit, which is a break of slope, the sea bot- 

 tom drops more rapidly and all the way to the flatter floor of 

 the deep sea, with gradients reaching as much as 1 in 10 and 



