A412 J. D. Dana on some modern Calcareous Rock-formations. 
reef of Oahu there are numerous caverns but a few feet below 
the surface, and they abound in calcareous depositions. But such 
facts are too well known to require enumeration. 
The possibility of any such depositions taking place from the 
sea is denied by Prof. Horsford as well as the presence of car- 
bonic acid in sea water. It is not admitted that sea water wash- 
ing over a reef will take up carbonate of lime and deposit it 
again, and any such means as this of consolidating coral sands is 
consequently set aside. Darwin states a fact observed at Ascen- 
sion Island, bearing on this point which we here cite. 
In his Journal, p. 588, he says :—‘ Lieutenant Evans informs 
me that during the six years he has resided on this Island (Ascen- 
sion) he has always observed that in the months of October and 
November, when the sand [of a calcareous beach] commences trav- 
elling towards the southwest, the rocks which are situated at the 
end of the long beach become coated by a white, thick, and very 
hard calcareous layer. I saw portions of this remarkable deposit, 
which had been protected by an accumulation of sand. In the 
year 1831 it was much thicker than during any other period. It 
would appear that the water charged with calcareous matter, by 
the disturbance of a vast mass of calcareous particles only pat- 
tially cemented together, deposits this substance on the first rocks 
against which it impinges. But the most singular circumstance 
is, that in the course of a couple of months, this layer is either 
abraded or redissolved, so that after that period, it entirely disap- 
pears. It is curious thus to trace the origin of a periodical in- 
crustation, on certaiti” isolated rocks, to the motion of the earth 
with relation to the sun; for this determines the atmospheric cur- 
rents which give direction to the swell of the ocean, and th 
again the arrangement of the sea-beach, and this again the quan- 
tity of calcareous matter held in solution by the waters of the 
neighboring sea.” 
The author in his Geological Report, mentions different exam- 
ples of incrustations of carbonate of lime on seashores, and simi- 
lar cases are described by other authors. A single case will suffice. 
On the island of Oahu the isolated pebbles of a beach were ob- 
served by the writer to be milked over with a very thin calcareous 
incrustation, and there was a gradual passage to the agglutination 
of pebbles and grains by the process into a mass. Such facts not 
only prove that atmospheric and sea water may take up carbonate 
of lime when running over calcareous deposits, but that such 
dissolved lime may be deposited again on sea shores ; and that 
depositions are sometimes formed on shores from the spray of the 
sea, or from the evaporation of the waters upon the retreat of the 
tides. The calcareous beach sands of a coral island are in the | 
hae! position to be thus consolidated, and below an exterior 
material, the ‘‘ beach sand-rock” is found. 
