ive reef-constructing zoophytes
cease to live. If, then, the subsidence required to account for all the
existing atolls must have amounted to three or four thousand feet, or
even sometimes more, we are brought to the conclusion that there has
been a _slow_ and _gradual_ sinking to this enormous extent. Such an
inference is perfectly in harmony with views which the grand scale of
denudation, everywhere observable in the older rocks, has led geologists
to adopt in reference to upward movements. They must also have been
gradual and continuous throughout indefinite ages to allow the waves and
currents of the ocean to operate with adequate power.

The map constructed by Mr. Darwin to display at one view the
geographical position of all the coral reefs throughout the globe is of
the highest geological interest (see above, p. 351.), leading to
splendid generalizations, when we have once embraced the theory that all
atolls and barrier reefs indicate recent subsidence, while the presence
of fringing reefs proves the land to be stationary or rising. These two
classes of coral formations are depicted by different colors; and one of
the striking facts brought to light by the same classification of coral
formations is the absence of active volcanoes in the areas of
subsidence, and their frequent presence in the areas of elevation. The
only supposed exception to this remarkable coincidence at the time when
Mr. Darwin wrote, in 1842, was the volcano of Torres Strait, at the
northern point of Australia, placed on the borders of an area of
subsidence; but it has been since proved that this volcano has no
existence.

We see, therefore, an evident connection, first, between the bursting
forth every now and then of volcanic matter through rents and fissures,
and the expansion or forcing outwards of the earth's crust, and,
secondly, between a dormant and less energetic development of
subterranean heat, and an amount of subsidence sufficiently great to
cause mountains to disappear over the broad face of the ocean, leaving
only small and scattered lagoon islands, or groups of atolls, to
indicate the spots where those mountains once stood.

On a review of the differently-colored reefs on the map alluded to, it
will be seen that there are large spaces in which upheaval, and others
in which depression prevails, and these are placed alternately, while
there are a few smaller areas where movements of oscillation occur. Thus
if we commence with