STRUCTURE OF CORAL REEFS. 137 



may be some hundreds of yards ; over the bottom in these shal- 

 low waters are spread out the coral plantations, down to a 

 depth of 80 or 100 feet. Finally there is a rather abrupt de 

 scent to depths beyond the reach of an ordinary sounding-lead. 

 The great difference in the rapidity with which the water deep- 

 ens depends chiefly on the varied character of submarine 

 slopes. Shallow waters may extend out for miles, especially 

 off the prominent points or angles ; but it is more common to 

 meet with the opposite extreme — great depths within a few 

 hundred feet. 



The outer reef or coral platform is generally a little the 

 highest at its seaward margin, owing partly to the growth of 

 ordinary corals and other species on this part, and also to the 

 accumulations which naturally would there be piled up by 

 the waves and become cemented. This part is therefore first 

 laid bare by the retreating tide ; and though a tempting place 

 for a ramble, it is often a dangerous place on account of the 

 heavy breakers. There is not only greater height, but often 

 also a remarkably smooth surface to the reef-rock, looking as 

 if water- worn, and frequently a blotching of the rock with va- 

 rious shades of pink and purple. These colors and the smooth- 

 ness, as observed by Chamisso, are due to incrusting Nulli- 

 pores ; and to the same calcareous sea- weeds, as Darwin first 

 observed, is often owing the increased height. The material 

 of the incrusting plant is more solid than ordinary coral, for it 

 is without a pore ; and layer is added to layer until it has con- 

 siderable thickness. It is thus an important protection to the 

 reef against the wash of the waters. 



Darwin states that on Keeling's Island, the Nullipore bed 

 has a thickness of two or three feet and a breadth of twenty 

 feet. Nullipores are abundant on the Paumotu reefs. Still, 

 they are not essential to the formation or protection of an 



