252 CORALS AND CORAL ISLANDS. 



the corals to the reef material, the rate of increase would be- 

 come about l-80th of an inch per year. 



The specimen of Oculina diffusa, referred to on page 125, 

 weighs 44 ounces, which is five- sixths more than that of the 

 Ma^andrina, while the average diameter of the clump is the 

 same. The average annual increase would consequently cover 

 a circular area of seven inches diameter 1-1 8th of an inch 

 deep. And making the same allowances as above, the rate 

 for the year for the whole reef-grounds would be l-44th of an 

 inch. The specimen of Maeandrina mentioned by Major 

 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 Oculina 

 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 favorably 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 uncertainties, it is evident that a reef in- 

 creases its height or extent with extreme slowness. If the 



