CORALS AND CORAL REEFS. ly, 
John Murray says* :——“ An attempt was made [during the 
Challenger Expedition] to estimate the quantity of carbonate 
of lime in the form of calcareous Alge, Foraminifera, Ptero- 
poda, Heteropods, and pelagic Gasteropods, in the surface- 
waters. A tow-net, having a mouth 123 inches in diameter, 
was dragged for as nearly as possible half a mile through 
the water. The shells collected were boiled in caustic 
potash, washed, and then weighed. The mean of four 
experiments gave 2°545 grammes. If these animals were as 
abundant in all the depths down to 100 fathoms as they 
were in the track followed by the tow-net, this would give 
over 16 tons of carbonate of lime in this form in a mass of 
the ocean 1 mile square by 100 fathoms deep.” In a 
footnote to Appendix II. of the Third Edition of Darwin’s 
Coral Reefs, p. 284, is the following:—“I estimate that 
this amount of carbonate of lime is equivalent to a solid 
layer of the same area which is approximately ‘00009 of an 
inch thick. We may arrive at it thus: taking 2°7 as the 
specific gravity of carbonate of lime, we shall find the 
volume of 16 tons to be about 212°4 cubic feet, or 7°8 cubic 
yards. This has to be spread out over an area of 3,097,600 
square yards (the number of square yards in a mile) giving 
the above result.” It is the calcareous parts of these 
surface organisms which, after the death of the animals, 
form a slow, but steady, drizzle of lime, which is rained from 
the surface upon the sea floor, and which, in the course of 
long ages, gives rise to deposits of very considerable thickness. 
These deposits are of importance in relation to one of the 
theories of the origin of coral reefs to which further refer- 
ence will be presently made. 
Before passing on to the subject of the corals themselves, 
we must consider how marine organisms in general make 
use of the supplies of lime-salts which rivers bring from the 
land into the waters in which those organisms live. It used 
to be supposed that marine lime-secreting organisms ob- 
tained their lime direct from the bicarbonate. For several 
years past it has become increasingly-evident that such 
* «Structure and Origin of Coral Reefs and Islands,” Proc. Roy. Soc. 
Edin., vol. x. p. 508. 1880. : 
