276 CORAL FORMATIONS. 



miles beyond the island. Again the barrier-reef, running for 

 nearly iooo miles parallel to the North-East coast of Australia, 

 and including a wide and deep arm of the sea, forms a third 

 class, and is the grandest and most extraordinary coral forma- 

 tion in the world. 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 difficulty 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 subterranean 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 corals 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 ; 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 



