196 GEOLOGY [CHAP. IX 



Letter 533 continue to subside, any number of feet, but if the average 

 duration (from all causes of destruction) of reefs on the same 

 spot is limited, then after this limit has elapsed the reefs 

 would perish, and if the subsidence continued they would be 

 carried down ; and if the group consisted only of atolls, only 

 open ocean would be left ; if it consisted partly or wholly 

 of encircled islands, these would be left naked and reefless, 

 but should the area again become favourable for growth of 

 reefs, new barrier-reefs might be formed round them. As an 

 illustration of this notion of a certain average duration of 

 reefs on the same spot, compared with the average rate 

 of subsidence, we may take the case of Tahiti, an island of 

 7,000 feet high. Now here the present barrier-reefs would 

 never be continued upwards into an atoll, although, should 

 the subsidence continue at a period long after the death of 

 the present reefs, new ones might be formed high up round 

 its sides and ultimately over it. The case resolves itself 

 into : what is the ordinary height of groups of islands, of 

 the size of existing groups of atolls (excepting as many of 

 the highest islands as there now ordinarily occur encircling 

 barrier-reefs in the existing groups of atolls) ? and likewise 

 what is the height of the single scattered islands standing 

 between such groups of islands? Subsidence sufficient to 

 bury all these islands (with the exception of as many of 

 the highest as there are encircled islands in the present 

 groups of atolls) my theory absolutely requires, but no more. 

 To say what amount of subsidence would be required for 

 this end, one ought to know the height of all existing islands, 

 both single ones and those in groups, on the face of the 

 globe and, indeed, of half a dozen worlds like ours. The 

 reefs may be of much greater [thickness] than that just 

 sufficient on an average to bury groups of islands ; and the 

 probability of the thickness being greater seems to resolve 

 itself into the average rate of subsidence allowing upward 

 growth, and average duration of reefs on the same spot. 

 Who will say what this rate and what this duration is ? but 

 till both are known, we cannot, I think, tell whether we ought 

 to look for upraised coral formations (putting on one side 

 denudation) above the unknown limit, say between 3,000 and 

 5,000 feet, necessary to submerge groups of common islands. 

 How wretchedly involved do these speculations become. 



