112 ON CORAL REEFS AND ISLANDS. 



tion. la the latter case the finely comminuted matter is retained 

 beneath the quiet waters, in the former the rude action washes 

 it away. 



IV. The almost total absence of fossils from many parts of 

 the coral reef-rock, and generally from the shore sandrocks, is one 

 of the most striking facts here exemplified. These rocks are 

 formed in the midst of life, and out of the enduring remains of 

 animals ; yet fossils, (as shown at Metia and other elevated reefs.) 

 are often rare. 



This absence of organic remains characterizes almost invaria- 

 bly the drift sandrock. On Oahu, where this rock forms hills 

 thirty or forty feet in height above the reef-rock, not a fossil nor 

 a fragment of one was distinguished by us, neither of shells nor 

 corals. This fact had been previously remarked by some of the 

 intelligent residents, and it was a matter of dispute whether one 

 or two shells had not been found. These formations are but a 

 few rods from waters prolific with the productions of the sea, 

 and were made from them. 



An explanation of this peculiarity, is obvious on the principle 

 already discussed — the action of a triturating sea. Everything 

 washed towards the shores, is ground down by this action and 

 reduced to sand ; and a large part of the sand is worn out and 

 carried off by the sea ; or, being thrown up by the reef, is blown 

 inward by the winds. 



It is a natural inference from these facts, that the non-fossilif- 

 eious sandstones of our continents are no good evidence of the 

 absence or sparing diffusion of animal life in the seas about whose 

 shores they may have been formed. If this destruction of fos- 

 sils is so complete when the sands are of limestone, much more 

 rapid and thorough should it be when they are siliceous. As the 

 sea by its action bears off the finer material, and leaves only 

 what is in the condition of sand or a coarser material, the lime of 

 fossils might be almost wholly removed from among siliceous 

 sands, and hardly a trace remain which the chemist could 

 detect. 



V. 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 described appears to be a fit materia! for its produc- 

 tion ; and when dried it takes much the appearance of chalk. 

 This fact was pointed out by Mr. Darwin, and was suggested to 

 the writer by the mud in the lagoon of Honden Island. Still it 

 does not explain the main point ; for under all ordinary circum- 

 stances, this mud solidifies into compact limestone, instead of 

 chalk. This appears, moreover, to be the result which should 

 be expected. What condition then is necessary to vary the re- 

 sult, and set aside the ordinary process ? 



