Corals and coral islands.

398

CORALS AND CORAL ISLANDS.

In the raised coral rock of Oahu (p. 371) there are several
long winding horizontal chambers, some of which are the
sources of subterranean streams that open out on the shores
between the layers of the rock, or from the mouths of caverns.
These running waters, and others trickling from above, are
obviously the eroding agents that have made the caves.
As briefly remarked on page 194, caverns are still more remarkable on the island of Atiu, on which the coral reef-rock
stands at about the same height above the sea as on Oahu,
Rev. John Williams states that there are seven or eight of
large extent on the island. Into one he entered by a descent
of twenty feet, and wandered a mile in one only of its branches
without finding an end " to its interminable windings." He says,
"Innumerable openings presented themselves on all sides as
we passed along, many of which appeared to be equal in height,
beauty and extent to the one we were following. The roof, a
stratum of coral rock fifteen feet thick, was supported by massy
and superb stalactitic columns, besides being thickly hung with
stalactites from an inch to many feet in length; some of these
pendents were just ready to unite themselves to the floor, or to
a stalagmitic column rising from it. Many chambers were
passed through whose fretwork ceilings and columns of stalactites sparkled brilliantly, amid the darkness, with the reflected
light of our torches. The effect was produced not so much by
single objects, or groups of them, as by the amplitude, the
depth, and the complications of this subterranean world."
Other similar caves exist on the neighboring island of
Mauke.
The Bermudas and Bahamas are also noted for their caverns. The great height of the easily eroded drift sand-rock
gives these reef-regions a chance for caverns, large and small;
a notice of the Bermuda caverns will be found on page 224.