vi. j ON COHAL AND COEAL MEEFS. 123 



ward face has been formed all round the island, with 

 only such gaps as result from the outflow of rivers, in 

 the place of sally-ports. 



The structure of the rocky accumulation in the en- 

 circling reefs and in the atolls is essentially the same as 

 in the fringing reef. But, in addition to the differences 

 of depth inside and out, they present some other pecu- 

 liarities. These reefs, and especially the atolls, are 

 usually interrupted at one part of their circumference, 

 and this part is always situated on the leeward side 

 of the reef, or that which is the more sheltered side. 

 Now, as all these reefs are situated within the region in 

 which the trade-winds prevail, it follows that, on the 

 north side of the equator, where the trade- wind is a 

 north-easterly wind, the opening of the reef is on the 

 south-west side: while in the southern hemisphere, where 

 the trade- winds blow from the south-east, the opening 

 lies to the north-west. The curious practical result 

 follows from this structure, that the lagoons of these 

 reefs really form admirable harbours, if a ship can only 

 get inside them. But the main difference between the 

 encircling reefs and the atolls, on the one hand, and 

 the fringing reefs on the other, lies in the fact of the 

 much greater depth of water on the seaward faces of the 

 former. As a consequence of this fact, the whole of 

 this face is not, as it is in the case of the fringing reef, 

 covered with living coral polypes. For, as we have 

 seen, these polypes cannot live at a greater depth than 

 about twenty-five fathoms ; and actual observation has 

 shown that while, down to this depth, the sounding-lead 

 will bring up branches of live coral from the outer 

 wall of such a reef, at a greater depth it fetches to 

 the surface nothing but dead coral and coral sand. 

 We must, therefore, picture to ourselves an atoll, or an 

 encircling reef, as fringed for 100 feet, or more, from its 



