252 ELEMENTARY GEOLOGY 
animal origin are those made by the coral polyps, 
which build extensive reefs. 
The reef-building corals are animals which live in 
colonies. They cluster together, and many individuals 
combine to build one coral skeleton (Fig. 142). Thou- 
sands of these, with beautifully branching forms, or 
in solid and massive bunches, exist side by side; and 
in each colony there are many animals, each anchored 
Fig. 142. 
A colony of coral polyps, expanded above the solid stony mass. 
firmly in place and hungry for food. To thrive they 
must have extremely favorable conditions. 
The reef-building corals cannot live m abundance 
where the temperature is below 68°, and hence they 
are mostly confined to tropical or semi-tropical re- 
gions. They cannot thrive in water whose depth is 
more than 100 feet, and will perish if exposed to the 
air for any considerable length of time. So they de- 
velop only below the low tide level. Since the creat- 
