described hy Mr Darwin. 43 



to the proper depth. When the whole island had disappeared, 

 a group of isolated atolls, scattered over a space of 250 miles 

 by 150, would mark the place it occupied, and indicate its 

 figure. All the atolls would be built up to the level of low 

 water ; and while the last founded might be only two or three 

 fathoms deep, the first might be two or three hundred. In 

 this way, the lower hills might have their representative reefs 

 as well as the higher, though the creatures that construct them 

 can woi'k only at limited depths. 



Again, if the principle be correct, we would expect to find 

 occasionally an unsubmerged remnant of land (an island), ac- 

 companied with harrier reefs, in a region where subsidence was 

 going on, that is, amidst a group of atolls. Now, this occurs 

 in the Caroline Archipelago, and one or two other places. 

 Moreover, as the conditions necessary to the life of corals 

 (which are imperfectly known) may cease at some spots where 

 they once existed, we might also expect (admitting the prin- 

 ciple of subsidence) to find reefs, in which the coral being dead, 

 could not raise itself to the low water level. Such a case is 

 met with in the Great Chagos Eank, 90 miles by 70. It has 

 a border from 5 to 10 fathoms under water, a second border, 

 or inner ledge, about 16 fathoms under water, and its central 

 parts, consisting of mud, are from 40 to 50 fathoms deep. It 

 is conceived to be " a half-drowned atoll." 



In New Caledonia, as Mr Darwin observes, we seem to wit- 

 ness the effects of subsidence in actual progress. It is an 

 island 200 miles in length by 45 in breadth, quite straight, 

 and consisting of a single ridge of mountains. Now, the coral 

 reefs, which run parallel to its shores on the two sides, instead 

 of turning round the north end and uniting, as we would ex- 

 pect, continue in their original north-west direction for 150 

 miles beyond it in the open sea. The most probable explana- 

 tion of this anomaly is, that the reefs, in their northern pro- 

 longation, accompany a part of the ridge, which, owing to the 

 island having subsided, is note submarine, but consisted of dri/ 

 land at an earlier period when the reefs were founded. The 

 reefs, in short, follow the ancient line of the shore, a large 

 part of which is now under water, and the process of submer- 

 gence is perhaps still going on. 



Lands recently raised, or still risiiig from the ocean. — While 



