S. parallels. Within these limits there are numerous
reefs and islands formed of coral intermixed with the calcareous
skeletons of other animals, and their formation has long been a matter
of dispute among naturalists and geologists.

Coral formations may be classed as fringing or shore reefs, barrier
reefs and atolls. _Fringing reefs_ are platforms of coral rock extending
no great distance from the shores of a continent or island. The seaward
edge of the platform is usually somewhat higher than the inner part, and
is often awash at low water. It is intersected by numerous creeks and
channels, especially opposite those places where streams of fresh water
flow down from the land, and there is usually a channel deep enough to
be navigable by small boats between the edge of the reef and the land.
The outer wall of the reef is rather steep, but descends into a
comparatively shallow sea. Since corals are killed by fresh water or by
deposition of mud or sand, it is obvious that the outer edge of the reef
is the region of most active coral growth, and the boat channel and the
passages leading into it from the open sea have been formed by the
suppression of coral growth by one of the above-mentioned causes,
assisted by the scour of the tides and the solvent action of sea-water.
_Barrier reefs_ may be regarded as fringing reefs on a large scale. The
great Australian barrier reef extends for no less a distance than 1250
m. from Torres Strait in 9.5 deg. S. lat. to Lady Elliot island in 24
deg. S. lat. The outer edge of a barrier reef is much farther from the
shore than that of a fringing reef, and the channel between it and the
land is much deeper. Opposite Cape York the seaward edge of the great
Australian barrier reef is nearly 90 m. distant from the coast, and the
maximum depth of the channel at this point is nearly 20 fathoms. As is
the case in a fringing reef, the outer edge of a barrier reef is in many
places awash at low tides, and masses of dead coral and sand may be
piled up on it by the action of the waves, so that islets are formed
which in time are covered with vegetation. These islets may coalesce and
form a strip of dry land lying some hundred yards or less from the
extreme outer edge of the reef, and separated by a wide channel from the
mainland. Where the barrier reef is not far from the land there are
always gaps in it opposite the mouths of rivers or considerable streams.
The outer wall of a barrier reef is steep