254 ELEMENTARY GEOLOGY 
illustrations of places where corals are able to grow. 
There are hundreds of coral islands in the open 
Pacific. On the coast of Australia there is a great 
coral reef, known as the Great Barrier Reef, whose 
length is more than 1200 miles (Fig. 143). 
By their life and death, these animals are building up 
great beds of limestone, but they are doing it slowly, 
for it is estimated that the average rate of growth of 
such a deposit is not 
more than one or two 
feet a century. Not 
only are these creatures 
making limestone beds 
where they grow, but 
the action of the waves 
on the coral shore, rasps 
Caroline Island in the Pacific. A very off the minute frag- 
perfect atoll. 
ments of carbonate of 
lime, and these are transported to sea as a milky white 
sediment, which may settle to the bottom many miles 
from the reef. 
Coral reefs are of three kinds: (1) The Pringing 
feef, which exists as a fringe near the coast; (2) Bar- 
rier eeefs, which develop at a considerable distance 
from the shore; and (3) Atolls, which are circular, ring- 
like islands of coral in the open ocean, without any 
other land near by (Fig. 144). 
