RATE OF GROWTH OF CORAL REEFS. 215 



Hunt, is not here made the basis of a calculation, because we 

 have not the specimen for examination, and it is not certain 

 that the diameter stated by him was not the horizontal di- 

 ameter. 



These estimates from the Mceandrina clivosa and Ocuhfia 

 diffusa have this great source of uncertainty, that the growth 

 of the groups may not have been begun in the first year of the 

 fourteen. Further, the corals obtained by Major Hunt near 

 Fort Taylor, Key West, may not have been as favourably situ- 

 ated for growth as those of the outer margin of the reef. 

 Again, we have made no allowance for the carbonate of lime 

 that is supplied by the waters by way of cement, supposing 

 that this must come originally, for the most part, from the 

 reef itself. Besides, we have supposed, above, all the coral 

 reef-rock to be solid, free from open spaces ; and, further, it 

 is not considered that much of it is a coral conglomerate, in 

 which the fragments have their original porosity. 



On the other side, we have not allowed for loss of de'bris 

 from the reef grounds by transportation into the deep seas 

 adjoining, believing the amount to be very small. 



Whatever the uncertainues, it is evident that a reef increases 

 its height or extent with extreme slowness. If the rate of 

 upward progress is one-sixteenth of an inch a year, it would 

 take for the addition of a single foot to its height, one hundred 

 and ninety years, and iox five feet a tJiousaiid years. 



It is here to be considered, that the thickness of a growing 

 reef could not exceed twenty fathoms (except by the 'iQw fee 

 added through beach and wind-drift accumulations), even it 

 existing for hundreds of thousands of years, unless there were 

 at the same time a slowly progressing subsidence ; so that if 

 we know the possible rate of increase in a reef, we cannot infer 

 from it the actual rate for any particular reef ; for it may have 

 been very much slower than that. Without a subsidence in 

 progress, the reef would increase only its breadth. 



In order to obtain direct observations on the rate of increase 

 of reefs, a slab of rock was planted, by the order of Captain 

 Wilkes, on Point A^enus, Tahiti, and by soundings, the depth 



