GEOLOGICAL CONCLUSIONS. isi 
some coral on the shores when the eruption took place, as is evident 
from imbedded fragments in the tufa; but the reef containing the 
chalk appeared to have been subsequent in formation. ‘There is no 
certain proof yet ascertained of any connexion between the fires of 
the mountain and the formation of the chalk. 
The facts leave the subject of the origin of chalk still in uncertainty. 
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 con- 
ditions 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; 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 than 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 discussion 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 Professor 
Bailey, for infusoria and polythalamia, without detecting anything of 
this kind. It appeared to contain nothing organic. 
VII. The analyses have shown that ordinary corals consist mainly 
of carbonate of lime, (p. 90.) There is a small proportion of fluorids 
and phosphates, with some silica, alumina, and oxyd of iron. These 
fluorids and phosphates, existing in the coral, must exist also in the 
limestone rock made from coral. It is probable from some trials made 
by Prof. Silliman, Jr., that these constituents may be found also in 
many shells. 
From the several analyses of corals by Mr. Silliman, we infer that 
