ee ee ee ee ee 
IN THE NEW BRITAIN GROUP. 89 
It is, however, far less impure than the “chalk mud” of the 
Addautie, es — ve 5 ince by Professor Sir bm ae Wyville 
omson, F.R.S., in 
substances. 
The same author mentions that the typical ager is free from 
silica, and so it would appear to be ee e above quoted 
analyses ; but the “insoluble rock debris” < of Ko ae he wagons 
David Forbes, F. R. S., pro agar consisted largely of si 
find any reference occurs in Professor Dana’s work on “ Corals 
and Coral Islands.’’ See p. 308. But this even is not true 
chalk ; it is merely a recent limestone derived from disinbapeel 
corals, and whie sapesinios chalk. 
r. Dana there 
mi The formation of chalk from coral is known to be exemplified 
at only one spot among the reefs of the Pac 
The coral mud often rege as if it might be a fit material for 
its production. Moreover, when simply dried, it has much the 
appearance of chalk, a fact pointed out by h pries w? Ngee in 
his memoir on the Bermudas (1834), and also by Mr. Darwin, 
and suggested to the author by the mud in the lag of Honden 
Island. ey this does not explain the origin of chalk, for, under 
all circumstances, this mud solidifies into compact lime- 
rn Santee of chalk, a result which would be naturally 
What condition then is necessary to vary the result 
il set aside the ordinary process ! 
“The only locality of chalk among the reefs of the Pacific, 
referred to above, was Py found on any of the coral islands, but 
in the siesvated reef of Oahu, near Honolulu, of which reef it 
forms a constituent sift It is 20 or 30 feet in extent, and 8 or 
10 feet deep. 
“The rock could not be distinguished from much of the chalk 
of England ; it is equally fine and even in its texture, as earthy 
in its fracture, and so soft as to be used on the blackboard in the 
native schools. 
“Some imbedded shells look precisely = enalk fossils. It 
contained, according to Professor Silliman, 92°80 per cent. of 
carbonate of lime, 2°38 of carbonate of © smarts besides some 
a, iitile of iron, silica, &e. 
“The locality is situated on the shores, quite above ihc aceoars 
level, near the foot of Diamond Hill. This hill is an extin ct tufa 
<— nearly 700 feet in height, rising from the water's dee: and 
ts origin it must have been partly ——— li is one of 
the lateral cones of Eastern Oahu, and was thrown up at the 
