Coral Reefs and Islands. 157 



that its height and steepness of submarine slopes are due to 

 the fact that its outflows of lava have kept ahead of the sub- 

 sidence, and also built up nearly 14,000 feet above the sea. 



This height is large, but the mean pitch of the sides of the 

 volcanic mountains of the island is between 5° and 7° 45' ; and 

 hence it is only the height which successive outflows should 

 have produced over a vent at the sea-level; and it may be 

 that the accumulation above tide -level has been made since 

 the supposed subsidence ceased. The depth of 2875 fathoms 

 found by the ' Challenger ' 40 miles east of Hawaii shows a 

 mean submarine slope to that point of 4° 30', as if here also 

 was a slope made by flowing lava. But more soundings are 

 needed to prove that the slope is a gradual one. 



14. The facts reviewed show the uncertainty of evidence as 

 to little or no subsidence, or as to recent elevation, from (1) 

 narrow reefs, or from (2) the volcanic character of islands, and 

 leave untouched the evidence of actual subsidence from the 

 features of barrier- and atoll-reefs and from deeply indented 

 coasts. 



15. After the above considerations, it is clear that the 

 theory of subsidence meets well the facts as to the varying 

 extent of reef among reef-bordered high islands. According 

 to it, (1) steepness of submarine slope may characterize the 

 side of a barrier-reef (as well as of an atoll) fronting east or 

 west, north or south, as is true of high islands ; but it is least 

 likely to occur in the direction of the trend of the island or 

 group, or that of current drift. (2) Fringing-reefs, or no 

 reefs, may characterize one side, that of bold bluffs, and wide 

 barriers the opposite. (3) The barrier- reef may be made on 

 the submarine slopes of the land, or on a broad plateau or 

 lowland area between ranges of elevations, one or both of 

 which have disappeared in the subsidence. (4) By continued 

 subsidence, the side having a fringing-reef or no reef, may, 

 later in the subsidence, be that of a very broad barrier-reef, 

 because of the form of the surface of the subsiding land ; and 

 vice versa. 



The third of these propositions is well illustrated by the 

 facts from the Maldives, as reported by Darwin. On account 

 of its importance I add an illustration from the Feejees. 



The great reef-grounds along the north-west sides of the 

 two large Feejee islands, Vanua Lebu and Viti Lebu, do not 

 indicate a subsidence proportional to their width. 



Each of these islands is over ninety miles long, and to- 

 gether the trend is north-eastward*. The north-western 



* A map of the Feejee group is contained in my ' Corals and Coral 

 Islands,' and of larger size in Wilkes's ' Narrative of the Expedition.' 



