Hicks on the (Reef=(Biulders. 299 
of the calcareous matter secreted by certain animals? And 
what else is the coral reef ? The polyps take vip carbonate of 
lime from the water and build it into masses of coral which are 
then broken, triturated and pulverized bv the waves, and finally 
deposited like any other sediment. 
In answer to this three reasons may be given for removing 
coral formations from the category of ordinary calcareous sedi- 
ments. In the first place the limited range in depth — the bath- 
ymetric limit — of the polyps, imposes peculiar conditions. 
Secondh', the polyps are true reef-builders. Sediment is 
something that settles down', the reefs are built tip. Not 
the whole of the reef indeed, but certainly that portion be- 
tween the depth of 120 feet^ and the line of wave action is a 
genuine building- of which the polyps are the architects and 
masons. In the third place, the pure calcareous matter, when it 
has been comminuted by the waves, hardens with such rapidity 
as to introduce a new and distinguishing feature in sharp con- 
trast with ordinary sedimentation. This facilty in passing from 
the condition of incohereiit fragments to that of firm and com- 
jolete consolidation, has two notable results. It prevents that 
dissipation of the rock material over wide spaces which would 
necessarily follow if solidification were long delayed; and it 
produces steep slopes instead of the horizontal planes which are 
characteristic of ordinary sediments. Even the loose coral sand 
will lie at almost as high an angle under water as dry sand in 
air. I have ascertained experimentally that the average resting 
slope of fine dry coral sand is 35° , and Prof. Agassiz observed 
slopes of 33° composed of coral sand under water. 
But notwithstanding these three good reasons for separating 
coral reefs from ordinary sediments it is certain that, in many 
cases, the best light in which to study them is that which is 
furnished by the ordinary laws of sedimentation. The Florida 
reefs, for instance, have been shown by Prof. Alexander Agas- 
siz to owe their form and their extension westward to the ac- 
tion of tides and currents which shape the calcareous sedi- 
1 There is much variance in the statements of the depth at which the 
reef-building species will live. Undoubtedly they will live much deeper 
in some seas than in others. I give the figures 120 ft. as a general av- 
erage. 
