GEOLOGICAL CONCLUSIONS. ~\N 4395 
The coral mud often looks 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 Lieutenant Nel- 
son in his Memoir on the Bermudas (1834), and also by Mr. 
Darwin, and suggested to the author by the mud in the lagoon 
of Honden Island. Still this does not explain the origin of 
chalk ; for, under all ordinary circumstances, this mud _solidi- 
fies into compact limestone instead of chalk, a result which 
would naturally be expected. What condition then is neces- 
sary to vary the result, and set aside the ordinary process. 
The one locality of chalk among the reefs of the Pacific, 
referred to above, was not found on any of the coral islands, 
but in the elevated reef of Oahu, near Honolulu, of which reef 
it forms a constituent part. It is twenty or thirty feet 
in extent, and eight or ten feet deep. The rock could not be 
distinguished from much of the chalk of England : it is equal- 
ly 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 like chalk fossils. It con- 
tained, according to Professor Silliman, 92°80 per cent. of 
carbonate of lime, 2°38 of carbonate of magnesia, besides 
some alumina, oxyd of iron, silica, ete. 
The locality is situated on the shores, just above high-tide 
level, near the foot of Diamond Hill. This hill is an extinct 
tufa cone, nearly seven hundred feet in height, rising from the 
water’s edge, and in its origin it must have been partly sub- 
marine. 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 
imbedded fragments in the tufa; but the reef containing the 
chalk appeared to have been subsequent in formation, and 
