ATOLL FORMATION 103 



ocean currents to form a submarine bank, no fringing 

 reef forming while the process is taking place, owing 

 to sediment produced by abrasion. On such a bank 

 an atoll would be built up owing to the more rapid 

 growth of corals on its rim, where they would be better 

 nourished. Submarine eruptions may occur anywhere, 

 and the contact of the molten lava with the water 

 produces a mound of cinders and ash, which may form 

 an island or never appear above the surface of the sea 

 (Fig. 32). Such a mound would rapidly be cut down 

 to the limits of wind- wave action, perhaps even to 

 40 fathoms or more, on which an atoll would directly 

 arise, in the way above suggested. Such a mound was 

 Falcon Island, Tonga, erupted in 1883 to a height of 

 about 250 feet, but now a submarine plateau covered 

 by a few fathoms of water. 



Thirdly, mounds might be built up beneath the sea 

 to suitable depths for the reef builders by the remains 

 of various organisms. On any slight elevation on the 

 ocean floor sedentary and other animals congregate to 

 an extraordinary degree. Chief among these are the 

 deep-sea corals, enormous banks of which have been 

 discovered on the floor of the Atlantic by dredging 

 and sounding expeditions. Their remains, assisted by 

 the remains of pelagic organisms, build up the surface 

 of the mound to an extent out of all proportion to the 

 upgrowth of the surrounding area of the ocean floor, 

 so that the mound ultimately approaches the surface 

 (Fig. 31). The reef builders obtain sway, and a reef 

 soon reaches the surface, later to spread out on its 

 own talus and to be hollowed out in its centre, forming 

 a typical atoll. 



In many coral-reef regions boats are often difficult 

 to procure, and the traveller is driven to study the 

 low-lying islands which. are so characteristic of coral 



