CORALS AND CORAL REEFS. ihs3 
that temperature is the chief element in determining the 
depth to which the living portion of a coral reef may 
extend, there is no reason why true reef-building corals 
should not have commenced to build over the whole of these 
areas, on the summits of submarine elevations which were as 
much as 600 feet below the level of the sea. If this could be 
proved to be really the case, a vertical thickness of 600 feet 
of coral reef could no longer be accepted as proof of 
subsidence. These are well-known and most important 
factors in the distribution of ocean-surface temperatures, and 
are therefore of vital importance in connection with the 
distribution of these corals which need a temperature above 
68° Fahr. If the reader will take the trouble to compare a 
map showing the distribution of coral reefs with any good 
maps showing the ocean-surface isotherms, ocean currents, 
and prevalent winds (such as the maps in Mill’s Realm of 
Nature), it will soon become apparent how closely dependent 
the distribution of coral reefs is upon these factors, acting 
separately or in combination. 
A study of the distribution of the coral reefs in the Gulf 
of Mexico reveals the influence of another factor of import- 
ance in relation to the welfare and distribution of corals. 
Off the mouths of great rivers vast bodies of fresh-water 
enter the sea. Numerous marine animals are very sensitive 
to the influence of even brackish water, and some are killed 
immediately by even a small reduction of salinity of the 
water in which they are accustomed to live. Again, with 
the outflow of river-water, vast quantities of liquid mud are 
transported seawards, giving rise to turbid water even far 
from the land. Both of these factors are inimical to the 
well-being of reef-building corals. Accordingly we find that 
it is only where the sea-water is pure and unmixed with 
sediment that reef-building corals can thrive. Consequently, 
although coral reefs are abundant around the West Indies, 
they are absent from the greater part of the Gulf of Mexico, 
because of the muddy water poured into the Gulf from the 
Mississippi and other rivers. 
Lastly, we have to consider the important question of the 
influence of abundance, or otherwise, of food-supply, in 
determining the distribution of corals. It must be remem- 
