On Coral Reefs and Islands. 79 
rapid and thorough should it be when they are siliceous. As-the 
Sea by its action bears off the finer material, and leaves only 
what is in the condition of sand or a coarser material, the lime of 
fossils might be almost wholly removed from among siliceous 
— and hardly a trace remain which the chemist could 
etect. 
V. The formation of chalk from coral is known to be exem- 
plified at only one spot among the reefs of the Pacific. The 
coral mud described appears to be a fit material for its produc- 
tion; and when dried it takes much the appearance of chalk. 
This fact was pointed out by Mr. Darwin, and was suggested to 
the writer by the mud in the lagoon of Honden Island. Still it 
does not explain the main point; for under all ordinary circum- 
stances, this mud solidifies into compact limestone, instead of 
chalk. This appears, moreover, to be the result which should 
be expected. What condition then is necessary to vary the re- 
sult, and set aside the ordinary process ? 
hoe islands, but in the elevated reef of Oahu, of which reef it 
Carbonate of lime, . j A 92:800 
Carbonate of magnesia, ‘ ; 2:385 
Alumina, . : j : 0250 
Oxyd of iron, . : , .. 0-643 
Silica, , 0:750 
Phosphoric acid and fluorine, . i 2-113 
_ Water and loss ‘ . 1-148 
The locality is situated on the shores just above high tide 
level, near: the foot of Diamond Hill. This hill is an extinet 
we It is one of the lateral cones of eastern Oahu, and was 
thrown Up at the time of an eruption through a fissure, the lavas 
of which appear at the base. There was some coral on the 
shores when the eruption took place, as is evident from imbed- 
ded fragments in the tufa; but the reef containing the chalk ap- 
Peared to have been subsequent in formation. ‘There is no cer- 
tain proof yet ascertained of any connection between the fires of 
the mountain and the formation of the chalk. 
i i aoe a 
i. oy 2 ae 'y 
