i 7 o THE SEAS 



surrounds the formation of the other two types of reefs. 

 Let us first assume the presence of a coastline, ideally 

 situated for the growth of corals but which, perhaps because 

 great earth movements have only recently raised it above 

 the surface of the sea, has no growth of corals. Corals 

 establish themselves in depths of thirty fathoms and less, 

 and then proceed to grow upwards in the manner illustrated 

 in Figure 35. When they reach low-water mark, upward 

 growth ceases because corals cannot withstand exposure 

 to the air for any but very short periods. Unable to 

 continue upwards, the corals must grow outwards, and so 



„ B 



A" 



Fig. 35. — Diagram showing formation of a Fringing Reef; A-B, original 

 slope of shore ; C, level of sea ; F, boulder zone at outer edge of reef ; 

 H, shallow passage between reef and land ; Y, coast eaten into by waves ; 

 dotted area, Region of actual growing Coral ; crosses, Region of dead Coral; 

 shaded area, Land (adapted from Crossland). 



the coral mass becomes broader and also much steeper on its 

 outer slope. The reason for this is that, though the corals 

 near the surface could grow outward to an unlimited extent, 

 they are actually limited by the growth of the underlying 

 mass which can only extend outwards as far as the thirty 

 fathom line, since below this line light is insufficient for the 

 proper nutrition of their associated plants. Hence they 

 stop growing while the mass above continues till a steep 

 precipice is formed on the outer side of the reef, down 

 the side of which fall coral boulders and debris of all kinds, 



