296 © 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 cir- 
cumstances 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 
