FORM OF INDIVIDUAL REEF. 315 



pacted together into a soft spongy sort of stone. 

 The greater part of the surface of this mound is 

 quite flat and near the level of low water. At its 

 edges it is commonly a little rounded off, or slopes 

 gradually down to a depth of two, three, or four 

 fathoms, and then pitches suddenly down with a 

 very rapid slope into deep water, 20 or 200 fathoms, 

 as the case may be. The surface of this reef, when 

 exposed, looks like a great flat of sandstone with a 

 few loose slabs lying about, or here and there an 

 accumulation of dead broken coral branches, or a 

 bank of dazzling white sand. It is, however, 

 chequered with holes and hollows more or less deep, 

 in which small living corals are growing ; or has, 

 perhaps, a large portion that is always covered by 

 two or three feet of water at the lowest tides, and here 

 are fields of corals, either clumps of branching madre- 

 pores, or round stools and blocks of maeandrina and 

 astrsea, both dead and living. Proceeding from this 

 central flat towards the edge, living corals become 

 more and more abundant. As we get towards the 

 windward side, we of course encounter the surf of the 

 breakers long before we can reach the extreme 

 verge of the reef, and among these breakers we see 

 immense blocks, often two or three yards (and some- 

 times much more) in diameter, lying loose upon 

 the reef. These are sometimes within reach by a 

 little wading ; and though in some instances they 

 are found to consist of several kinds of corals matted 

 together, they are more often found to be large in- 



