80 On Coral Reefs and Islands. 
The facts leave the subject of the origin of chalk still in un- 
certainty. Its fine earthy texture is evidence that the deposit 
was not subaerial seashore accumulation, as only sandstones and 
conglomerates, with rare instances of more compact rocks, are 
thus formed. Sandrock making is the peculiar prerogative, the 
world over, of shores exposed to waves, either marine or fresh 
water. We should infer, therefore, that the accumulations were 
produced either in confined areas, into which the fine material 
from a beach may have been washed, or on the shores of shallow, _ 
quiet seas: 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 occu 
pied by it. ‘That there were some peculiar cireumstances distil 
guishing this from other parts of the reefs, is evident; and this 
appears to be the only probable supposition. If this be admitted, 
the existence of an elevated temperature might be suggested for 
certain areas during the deposition of the chalk strata. It is well 
known that heated waters dissolve lime much less readily that 
cold; and this might be a reason for its inferior hardness and 
earthy texture. ‘The character of the cretaceous deposits presents 
‘many interesting points bearing upon this subject; but a discus 
sion of them would be out of place here, as our object is simply 
to state such inferences as the facts observed among existing reefs 
may have suggested. 
This coral chalk has been examined microscopically by Pro 
fessor Bailey, for infusoria and polythalamia, without detecting 
anything of this kind. It appeared to contain nothing organi. 
VI. The analyses have shown that ordinary corals consist 
mainly of carbonate of lime. There is a small proportion of 
fluorids and phosphates, with some silica, alumina, and oxy4 
of iron. ‘These fluorids and phosphates, existing in the coral, 
must exist also in the limestone rock made from coral. It 
i ee 
that the fluorids and phosphates amount, on an average, to 
4th per cent., or 0°25 parts in a hundred of coral: and the amount 
sot . 
> fluorids are HU — 
phates, phosphates | 
“a 
ae ql 
dred and forty pounds of phosphates. These 
ds of calcium and magnesium, and the phos 
. = 
