3o8 CORALS AND CORAL ISLANDS. 



X. FORMATION OF CHALK. 



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 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 Nelson 

 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 only 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 ex- 

 tent, and eight or ten feet deep. The rock could not be dis- 

 tinguished 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 like chalk fossils. It 

 contained, according to Professor Silliman, 92-80 per cent, of 

 carbonate of lime, 2*38 of carbonate of magnesia, besides 

 some alumina, oxide of iron, silica, &c. 



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 



