256 CORALS AND CORAL ISLANDS. 
the corals to the reef material, the rate of increase would be. 
come about 1-80th 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 
Meeandrina, while the average diameter of the clump is the 
same. The average annual increase would consequently cover 
a circular area of seven inches diameter 1-18th of an inch 
deep. And making the same allowances as above, the rate 
for the year for the whole reef-grounds would be 1-44th of an 
inch. The specimen of Meandrina 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 
diameter. For other facts see the Appendix. 
These estimates from the Maandrina 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 dé- 
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 

