Coral reefs often extend to a depth of three hundred feet below the surface of the ocean, and formerly persons were puzzled to know how they could have grown in such deep water, as no coral polyps can live at a greater depth than twenty or thirty fathoms. This puzzling question was settled by the late Charles Darwin, who first showed that coral islands occur where there has been a gradual sinking of the bottom of the ocean. As the reef rises in height, the sinking of the foundation partly counteracts the upward growth of the coral; consequently the proper depth of water is secured, and the reef appears to be stationary, whereas it is really growing upward.
Whenever a coral reef rises above the surface of the ocean, we may know that the coral, which grew under water, has been lifted above the level of the sea by a rising of the ocean-bed.
These circular reefs are called "atolls." They are quite different from the "fringing reefs," which extend along the shores of continents and islands. There are usually openings or breaks in fringing reefs directly opposite the mouths of rivers and fresh-water streams, as the corals can not endure currents which carry mud or sediment. Perhaps the grandest reef to be found in any part of the world is the one extending along the northeast coast of Australia. It is nearly one thousand miles in length, and proves to us that the helpless coral polyps have played no trifling part in the formation of our earth. All they have accomplished has been done merely by their living and growing.
WINTER.