290 Dr. Mac Culloch on the 
shoals, and the rain falls into it, it at length becomes freshened, 
so that the animals die, and the operation of filling it up ceases. 
Thus it becomes a fresh-water lake, and forms {that receptacle 
which is so common a feature in all the flat islands of those seas. 
Of whatever size the circle may be, but particularly if it be 
large, the islands begin first to collect on the outside of the reef, 
while within it, projecting parts, or banks and rocks, are scat- 
tered in different places. The ridge, or dam, to windward, 
under the protection of which the whole mass extends, is pro- 
duced by the fragments of the corals. Whenever they have 
arrived at the surface of high-water mark, they cease to grow 
any longer, as the animal cannot live out of the water. But at 
low water, the reef is of course above the sea. Thus the force 
of the waves breaks off the upper parts, and washes them 
onwards to leeward, where they collect ; while the animals, still 
working upwards on the windward side, keep up a constant 
supply of materials destined to the same end. Thus a bank of 
dead matter, or of fragments and sand, produced by the wear 
of the corals, is formed on the top of the living rock, and ce- 
mented by the solvent power of the water on the carbonate of 
lime. In this manner, it is raised above the level of the high- 
water mark, and kept smooth by the surf which continually 
breaks over it, until it is elevated even beyond the reach of 
the sea. The sand and fragments in time consolidate, so as 
to produce regular strata, resembling the calcareous rocks of 
Guadaloupe, Bermuda, Bahama, and other West India islands ; 
and fragments of these, forming large blocks of stone, are fre- 
quently piled up in the ridge, and even further onwards, till a 
large extent of surface becomes thus consolidated by the aid of 
more sand and fragments, and sometimes by that of shells also, 
into a solid mass of land. As the same process is also going on 
in the interior parts, where the projecting banks lie, all these at 
length extend and unite ; so that islands of any magnitude may 
in this manner at length be produced. Occasionally, the lakes 
before-mentioned, are also filled up by the growth and decom- 
position of vegetables, becoming first marshy spots, and at 
jJength dry land. Had it not become a sort of fashion in phi- 
