CORAL REEFS. 143- 



(PI. X., Fig. 2). Some of these had been long ago recognised by 

 Captain Cook as formed by wave action upon the detritus derived 

 from the coral reefs. All of those that I saw were certainly of this 

 origin, and correspondents living in this group of islands have been 

 good enough to inform me that though they have landed on a 

 great many islets on the barrier reef on either side of ship passes, 

 they do not know of one that is formed of volcanic rock. 



I was not able to make any observations that would bear on 

 the questions whether solution had or had not taken place on the 

 reef masses to any extent, but it is important to note that the 

 fringing reef of the present day situated on the land side of the 

 lagoon consists still of living coral. 



It will be seen from the chart that Tahiti evidently is rather 

 different from the other islands, as its lagoon is less continuous and 

 less deep, and the fringing reef and barrier are in many places 

 united. Darwin maintains that this points to a long continuance 

 of the present elevation. Agassiz, on the other hand, maintains 

 that the level has changed so recently that the effect of solution has 

 not yet become pronounced. 



There certainly seems in the presence of the wide alluvial plains, 

 and of the stream valleys filled with detritus, strong geological reason 

 to agree with Darwin in this question. That Darwin did not neglect 

 the possibility of the action of marine erosion is well seen in the 

 following extract. : — 



Darwin. "Voyage of Beagle," p. 455. J. M. Dent, "Every- 

 man's Library. Barrier Reefs." "Are we to suppose that each 

 island is surrounded by a collar-like submarine ledge of rock or by a 

 great bank of sediment ending abruptly where the reef ends. If 

 the sea had formerly eaten deeply into the islands before they were 

 protected by the reefs, thus having a shallow ledge all round them 

 under water, the present shores would have been invariably bounded 

 by great precipices, but this is most rarely the case." 



Much emphasis has been laid on the presence of raised coral in 

 the neighbourhood of atolls or barriers. It does not appear that 

 this is necessarily of great importance, for the lapse of time within 

 even the most recent geological periods is sufficient to allow of the 

 elevation of one reef and subsequently of depression from an ele- 

 vated position, during which the atoll or barrier may have been 

 formed. 



In regard to this question Agassiz quotes with approval the 

 remark of Dana. 



Agassiz, loc. cit., p. xxi. 



Dana very justly remarks ' It is important to have in mind 

 that the coral reef era probably covered the whole of the quaternary 

 and perhaps the Pliocene tertiary also, and hence the local eleva- 

 tions that may have taken place were not crowded events of a 

 short period.' " 



In many instances Agassiz states that reefs are based upon a 

 platform of eroded tertiary coralliferous limestone. This appears 



