﻿532 -P^'o/l Limrsidge — Chalk in New Britain. 



rock debris" mentioned by the late talented David Forbes, F.R.S., 

 probably consisted largely of silica. 



The only locality for chalk in the Pacific Islands to which I can find 

 any reference occurs in 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 disintegrated corals, and which resembles chalk. 



"The formation of chalk from coral is known to be exemplified 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 I^elson 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 

 solidifies into compact limestone instead of chalk, a result which would 

 be naturally expected. What condition then is necessary to vary the 

 result, and set aside the ordinary process ? 



*' The only locality for 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 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 con- 

 tained, 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, etc. 



" The locality is situated on the shores, quite above high-tide level, 

 near the foot of Diamond Hill. This hill is an extinct tufa cone, 

 nearly 700 feet in height, rising from the water's edge, and in its 

 origin it must have been partly submarine. It is one of the lateral 

 cones of Eastern Oahu, and was thrown up at the time of an eruption 

 through a fissure, the lava of which appears 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 afi'orded no 

 certain proof of any connexion between the fires of the mountain and 

 the formation of the chalk. 



" The fine earthy texture of the material is evidence that the deposit 

 was not a subaerial sea-shore accumulation, since only sandstones and 

 conglomerates, with rare instances of more compact rocks, are thus 

 formed. Sand-rock making is the peculiar prerogative, the world over, 

 of shores exposed to waves, or strong currents, either of marine or of 

 fresh water. We should infer, therefore, that the accumulation was 

 produced either in a confined area, into which the fine material from a 

 beach may have been washed, or on the shore of a shallow quiet sea — 

 in other words, under the same conditions nearly as are required to 

 produce the calcareous mud of the coral island. But, although the 

 agency of fire in the result cannot be proved, it is by no means im- 



