308 Geological Society : — Mr. Darwin on areas of 



The reef itself in the three classes, encircling, barrier and lagoon, 

 is most closely similar; the difference entirely lying in the absence 

 or presence of neighbouring land, and the relative position which 

 the reefs bear to it. The author particularly points out one diffi- 

 culty in understanding the structure in the barrier and encircling 

 classes, namely, that the reef extends so far from the shore, that a 

 line drawn perpendicularly from its outer edge down to the solid 

 rock on which the reef must be based, very far exceeds that small 

 limit at which corals can grow. A distinct class of reefs however 

 exists, which the author calls " fringing reefs," which extend only 

 so far from the shore, that there is no difficulty in understanding 

 their growth. The theory which Mr. Darwin then offered, so as to 

 include every kind of structure, is simply that as the land with the 

 attached reefs subsides very gradually from the action of subterra- 

 nean causes, the coral building polypi soon again raise their solid 

 masses to the level of the water ; but not so with the land : each 

 inch lost is irreclaimably gone : — as the whole gradually sinks, the 

 water gains foot by foot on the shore, till the last and highest peak 

 is finally submerged. Before explaining this view in detail, the 

 author offered some considerations on the probability of general 

 subsidences, — such as the small portion of land in the Pacific, where 

 many causes tend to its production, an argument first suggested by 

 Mr. Lyell, and the extreme difficulty (with the knowledge that co- 

 rals grow at but limited depths) in explaining the existence of a 

 vast number of reefs on one level, without we grant subsidence, so 

 that one mountain top should be submerged after another 5 the 

 zoophytes always bringing up their stony masses to the surface of 

 the water. Subsidence being thus rendered almost necessary, it was 

 shown by the aid of sections, that a simple fringing reef would thus 

 necessarily be converted by the upward growth of the coral into 

 one of the encircling order, and this finally, by the disappearance 

 through the agency of the same movement of the central land, into 

 a lagoon island. In the same manner a reef skirting a shore would 

 be changed into a barrier extending parallel to, but at some di- 

 stance from, the mainland. 



Mr. Darwin then showed, that there existed every intermediate 

 form between a simple well characterized encircling reef, and a la- 

 goon island ; that New Caledonia supplied a link between encircling 

 and barrier reefs ; that the different reefs produced by thesame order 

 of movement were always in juxtaposition, of which the Australian 

 barrier associated with encircled islets and true lagoons, affords a 

 good example. He then proceeded to show that within the lagoon 

 of Keeling Island, proofs of subsidence might be deduced from 

 many falling trees and a ruined storehouse ; these movements ap- 

 pearing to take place at the period of bad earthquakes, which like- 

 wise affect Sumatra, 600 miles distant. It was thence inferred as 

 probable, that as Sumatra rises, (of which proofs are well known to 

 exist,) the other end of the lever sinks down ; Keeling Island thus 

 acting as an index of the movement of the bottom of the Indian 

 Ocean. Again at Vanikoro, where the structure indicates accord- 



