J.D. Dana on some modern Calcareous Rock-formations. 417 
_ powers by polarized light shows, as the writer has observed, no 
distinct crystalline texture. 
If coral zoophytes require the medium of carbonate of ammo- 
nia, so must also mollusca, as they derive their carbonate of lime 
from the same waters; and so also fishes and Cetacea, except so 
far as they may receive it from animals taken as food. The pro- 
cess is essentially alike in all. 
Although the presence of carbonate of lime in sea water about 
coral reefs cannot be doubted, it is still true that the lime is mainly 
in the state of sulphate. ‘The writer has presented evidence on 
this point in his Report which is overlooked by Prof. Horsford. 
_ He adds a few forms that were 
obtained from the evaporation 
of a single drop of sea water 
under the microscope while en- ~ A 
gaged in examining a minute | , 
Crustacean. ‘The re-solution wet ee 
and re-crystallization of gypsum was often repeated in his micro- 
Scopic examinations. 
In view of the above considerations, we may refuse our assent 
essential to the coral secretions. he carl 
produced by animal excretions may, by this double decompo- 
Sition, be one source of carbonate of lime in the sea; but if so, 
this carbonate of lime should abound most about coral reefs, as 
it could not be wholly abstracted by the animals from the moving 
Waters. 
A few other statements in Prof. Horsford’s paper require a 
brief remark. 
Prof. Horsford states on page 249, “ that the exceeding fineness 
of the coral mud is due to the stone-plants which flourish in the 
waters within the reef and which admit of ready reduction to @ 
lanation, as h 
appeared to the writer,* is, that it is owing 
whatever coral fragments and shells may be deposited in te 
where the motion of the water is gentle, while the coarser 
and pebbles are found where the waters are St 
lent agitation, as in the face of the breakers. ‘The geen vm 
happen under like circumstances if the rock material were g oa 
ite instead of coral. The corals within most lagoons are as soli 
_as those elsewhere formed. 
* Expl. Exped. Geol. Rep. p- 107, and this Journal, [2], xii, 333. 
