Corals and coral islands.

396

CORALS AND CORAL ISLANDS.

afforded no certain proof of any connection between the fires
of the mountain and the formation of the chalk.
The fine earthy texture of the material is evidence that the
deposit was not a subaerial seashore accumulation, since only
sandstones and conglomerates, with rare instances of more
compact rocks, are thus formed. Sand-rock making is the
peculiar prerogative, the world over, of shores exposed to
waves, or strong currents, either of marine or fresh water. We
should infer, therefore, that the accumulation was produced
either in a confined area, into which the fine material from a
beach may have been washed, or on the shore of a shallow,
quiet sea; in other words, under the same conditions nearly
as are required to produce the calcareous mud of the coral
island. But, although the agency of fire in the result cannot
be proved, it is by no means improbable, from the position
of the bed of chalk, that there may have been a hot spring at
the spot occupied by it. That there were some peculiar circumstances distinguishing this from other parts of the reefs, is
evident.
This, if a true conclusion, is to be taken, however, only as
one method by which chalk may be made. For there is no
reason to suppose that the chalk of the Chalk formation has
been subjected to heat. On the contrary, it is now well ascer* tained that it is of cold-water origin, even to its flints, and that
it is made up largely of minute foraminifers, the shells of
Rhizopods. Professor Bailey found under his microscope
no traces of foraminifers, or of any thing distinctly organic,
in the Oahu chalk.
XI. RATE OF INCREASE OF LIMESTONE FORMATIONS.
On page 253 it is shown that coral-reef limestones are of
slow formation, the rate of increase in thickness, where all