CORALS AND CORAL REEFS. 7 
and his fellow-workers had given to the public. In one of 
my own papers above referred to, I endeavoured to show 
that, indirectly as well as directly, organisms may contribute 
to the deposition of carbonate of lime on the sea bottom. 
They convert solution of sulphate of lime into solid carbonate 
of lime while the animals themselves are living; and, when 
they are dead, the decomposition of the organic matter gives 
rise to a chemical change, whereby an additional quantity 
of the sulphate of lime present in sea-water is thrown 
down as a precipitate of carbonate of lime. It is in 
this way that much of the “paste” of many limestones is 
formed. 
An attentive consideration of these facts will make it 
clear that carbonate of lime must be in process of deposition 
through both organic and inorganic agencies over a large 
part of the floor of the ocean. The largest quantity is de- 
posited in these regions of the ocean where the surface- 
waters are warm, and the smallest quantity where they are 
cold. But although carbonate of lime is thus descending, 
from so large an area of the surface, in the direction of the 
bottom of the sea, the pressure of the ocean water in its 
greater depths tends to redissolve the carbonate of lime. 
Hence, what the organic agencies have fixed in the solid 
form from solution is again dissolved, and is returned to the 
general circulation. This result is modified by another 
cause: carbonate of lime, when crystallised, as it usually is 
in the organic state, assumes two different forms. One of 
these is orthorhombic, and the compound then forms the 
mineral Aragonite; the other form is rhombohedral, and is 
distinguished as Calcite. Organisms consisting of Aragonite 
begin to dissolve in sea-water after descending a few hundred 
fathoms; and although the dissolution is slow in the lesser 
depths, it is completed by the time that the organisms have 
descended to a depth of 1500 fathoms. Those of Calcite 
withstand the pressure better, and their solution is not 
completed until they have descended to a depth of 2900 
fathoms. Hence, in water of a greater depth than that last 
mentioned, little or no carbonate of lime reaches the bottom 
in the solid state; but becomes diffused in solution through 
the sea-water, where it is probably reconverted sooner or 
