CORALS AND CORAL REEFS. 15 
thrive and push their growth seawards on the outer margin 
of the mass. Hence, while in the earlier stages of growth of 
a coral reef, its form tends to become more or less that of 
an inverted cone, in its later stages of growth the tendency 
is to grow into a comparatively-thin sheet-——its thickness, in 
each case, being determined by the depth to which the 
temperature of 68° Fahr. extends—and in the stages of its 
growth later still, one may almost say that there is a tendency 
for the sheet of coral to sever itself from its original place 
of attachment, due to the fact that the older-formed portions 
of the reef have ceased to live, and are undergoing both slow 
solution and mechanical disintegration. It is this cause which 
gives rise to the channel between certain marginal reefs and 
the adjoining land, and also to the saucer-shaped medial de- 
pression in the case of atolls. 
The solvent action just mentioned is most potent at the 
surface, probably on account of the solvent, which is carbonic 
acid, being due to the decomposition of the abundant animal 
matter. The older and especially the deeper-seated portions 
of a reef, however, have often become converted by contact 
with the solutions of magnesia present in sea-water, into that 
double carbonate of lime and magnesia known as dolomite. 
Dolomite is very much less easily dissolved by water holding 
carbonic acid than is the pure carbonate of lime. Conse- 
quently, from the combined action of these two causes, there 
is much less solution of the lower parts of the reef than of 
those immediately below the surface. Even, however, if a 
certain amount of solution, as well as some disintegration, by 
the mechanical action of the sea, does take place, there are 
compensating causes at work in connection with the lower 
portions of a reef which quite make up for any loss that the 
reef may sustain in other ways. The compensating causes re- 
ferred to arise partly from what may be termed the accessory 
portions of the reef. Numberless animals of many kinds, 
and many lime-secreting plants, such as nullipores, harbour 
in the spaces between the branches of the corals, and their 
dead remains, together with broken portions of the corals which 
have been thrown on to the reef by the waves, tend to gravitate 
sooner or later to the lower parts of the reef, and seem to 
more than compensate by their aggregate bulk for the 
