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 to 150 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 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 
