80 Proceedings of Royal Society of Edinburgh. [sess. 
early voyagers into tlie seas of equatorial regions of the ocean. 
Questions connected with the peculiar form, the structure, the 
origin, and the distribution of these great natural productions have, 
from the very outset, puzzled and interested all those who delight 
in the study of natural things. In this communication w r e propose 
to point out and discuss some of the more general phenomena of 
oceanic deposits, with special reference to the functions of corals and 
other lime-secreting organisms, and the accumulation of their dead 
shells and skeletons on the floor of the great oceans. 
Coral reefs are developed in greatest perfection in those ocean 
waters where the temperature is highest and the annual range is 
least. It may he said that reefs are never met with where the 
temperature of the surface water, at any time of the year, sinks 
below 70° Fahr., and where the annual range of temperature is 
greater than 12° Fahr. Bermuda, which is the coral island the 
farthest removed from the equator,* and one or two other outlying 
reefs, may he, in a sense, exceptions to this statement, for in these 
exceptional cases the temperature of the ocean water appears occasion- 
ally to fall to 66° or 64° Fahr., and there is a wider annual range 
than 12° Fahr. This condition of high temperature with small 
range in the temperature of the water is only to he met with in the 
middle and western portions of the Atlantic and Pacific Oceans and 
the central parts of the Indian Ocean ; consequently, coral reefs 
flourish along the eastern shores of the continents where the coasts 
are bathed by currents of pure oceanic water coming directly from the 
open sea, while, on the other hand, they are absent along the western 
shores of the continents where the water is colder and the annual 
range is very much greater — for instance, off the western coasts of 
America and Africa. The “ Challenger ” observations have also 
shown that the layers of warm surface waters are much thicker 
towards the western parts of the great oceans ; consequently, reef- 
forming organisms flourish at a greater depth along the eastern 
shores of the continents than in positions further to the eastward 
in the open ocean, where the warm layer of water — over 70° Fahr. 
— is much thinner. Throughout the temperate and polar regions 
there are no coral reefs. This is all the more remarkable, seeing 
that organisms belonging to the same orders, families, and even 
Lat. 32° N. 
