358 CORALS AND CORAL LSLAND8. 



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 bv 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, etc. 



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 



