Coral Reefs and Islands. 161 



19. Elevated coral-reefs afford an opportunity to search for 

 layers of beach-made rock underlying true reef-rock ; and 

 also, if over 120 feet in height, to ascertain directly the cha- 

 racter of the rocks below this level. 



The elevated atoll, Metia, 75 miles north-east of Tahiti, 

 whose maximum height (according to the measurement 

 of officers of the Wilkes Expedition) is 250 feet, I have 

 described as consisting of the true coral-reef rock. My ex- 

 aminations were made on the west side, where it presents a 

 vertical front to the water. The white compact limestone 

 was, in some parts, almost destitute of fossils, or had only an 

 occasional mould of a shell or fragment of coral * ; and in 

 others it was a fine or coarse coral-breccia. My notes written 

 out at the island include the statement that u large masses of 

 corals make some lower layers." This observation, though 

 not as complete as I now see that it should have been, favours 

 the conclusion that the thickness of the reef rock is at least 

 twice as great as the depth to which reef- corals grow, in 

 which case the elevated reef is proof of a subsidence of 120 feet 

 or more. 



The island is so near the route to Tahiti that the doubt 

 which remains could be readily removed. 



20. The subsidence indicated, according to the Darwinian 

 theory, by atolls and barrier-reefs was actual, not apparent 

 subsidence attributable to change of water-level. The diffe- 

 rence in its amount between the Central- Pacific area of sub- 

 sidence and its limits (§§ 10, 11, above), the gradation or 

 variation in amount of subsidence along chains of islands 

 (§§ 10, 12, 13), and the local character of elevations, like those 

 of Metia, Mangaia, and many others, are proofs on this point. 



The preceding explanations have prepared the way for the 

 consideration of the arguments urged against the Darwinian 

 theory, to which I now pass. 



[To be continued.] 



conglomerate, whether consisting of rounded masses or angular, are 

 * deposited after life has become extinct/' no inference as to the parti- 

 cular kind of coral-rock intended can be drawn from the remark. From 

 my knowledge of the island I presume he meant the ordinary breccia 

 coDglomerate of the reef-rock, which is one of the kinds of coral-rock of 

 the elevated island. Commodore Wilkes himself made no examination 

 of the rock or special study of coral-islands, as might be inferred from 

 his theoretical views on p. 270 of volume iv. His i Narrative ' was to a 

 considerable extent made up from the journals of his various officers. 



* It was this compact rock, white, flint-like in fracture, clinking under 

 the hammer, that was found on analysis by B. Silliman to contain 38 07 

 per cent, of magnesium carbonate. 



Phil Mag. S. 5. Vol. 20. No, 123, August 1885, M 



