1H36.J COKAL FORMATIONS. 465 



These lioluthuriae, the fish, tlie numerous burrowino;' shells, and 

 nereidous ^^o^ms, which perforate every block of dead coral, 

 must be very efficient ag-ents in producing the fine wliite mud 

 wliich lies at the bottom and on tlie shores of the lap^oon. A 

 portion, however, of this mud, which when wet strikingly le- 

 sembled pounded chalk, w^as found by Professor Ehrenberg tO 

 be partly composed of siliceous-shielded infusoria. 



April 12th. — In the morning we stood out of the lagoon on 

 our passage to the Isle of France. I am glad we have visited 

 these islands : such formations surely rank high amongst the 

 wonderful objects of this world. Captain Fitz Roy found no 

 bottom with a line 7200 feet in length, at the distance of only 

 2200 yards from the shore ; hepce this island forms a lofty sub- 

 marine mountain, with sides steeper even than those of the most 

 abrupt volcanic cone. The saucer-shaped summit is nearly ten 

 miles across ; and every single atom,* from the least particle to 

 the largest fragment of rock, in this great pile, which however 

 is small compared with very many other lagoon-islands, bears 

 the stamp of having been subjected to organic arrangement. We 

 feel surprise when travellers tell us of the vast dimensions ct 

 the Pyram.ids and other great ruins, but how utterly insignificant 

 are the greatest of these, when compared to these mountains of 

 stone accumulated by the agency of various minute and tender 

 animals! This is a wonder which does not at first strike the 

 eye of the body, but, after reflection, the eye of reason. 



I will now give a very brief account of the three great classes 

 of coral-reefs ; namely. Atolls, Barrier, and Fringicg-reefs, and 

 will explain my views f on their formation. Almost every 

 voyager who has crossed the Pacific has expressed his unbounded 

 astonishment at the lagoon-islands, or as I shall for the future 

 call them by their Indian name of atolls, and has attempted 

 some explanation. Even as long ago as the year 1605, Pyrard 



* I exclude, of coiirse, some soil which has been imported here in vess^I« 

 from Malacca and Java, and likewise some small fragments of pumice, 

 drifted here by the waves. The one block of green-stone, moreover, on tbo 

 northern island must be excepted. 



t These were first read before the Geological Society in May, 1S37, 

 and have since been developed in a stparate volume on the 'Structure aud 

 Distribution of Coral Reefs.' 



