136 ON CORAL AND CORAL REEFS 



that of successive generations of coral polypes, and as 

 each generation takes a certain time to grow to its full 

 size, and can only separate its calcareous skeleton from 

 the water in which it lives at a certain rate, it is clear 

 that the reefs are records not only of changes in physical 

 geography, but of the lapse of time. It is by no means 

 easy, however, to estimate the exact value of reef- 

 chronology, and the attempts which have been made to 

 determine the rate at which a reef grows vertically 

 have yielded anything but precise results. A cautious 

 writer, Mr. Dana, whose extensive study of corals and 

 coral reefs makes him an eminently competent judge, 

 states his conclusion in the following terms : 



" The rate of growth of the common branching mad- 

 repore is not over one and a half inches a year. As the 

 branches are open, this would not be equivalent to 

 more than half an inch in height of solid coral for the 

 whole surface covered by the madrepore; and, as they 

 are also porous, to not over three-eighths of an inch of 

 solid limestone. But a coral plantation has large bare 

 patches without corals, and the coral sands are widely 

 distributed by currents, part of them to depths over one 

 hundred feet where there are no living corals ; not more 

 than one-sixth of the surface of a reef region is, in fact, 

 covered with growing species. This reduces the three- 

 eighths to one-sixteenth. Shells and other organic relics 

 may contribute one-fourth as much as corals. At the 

 outside, the average upward increase of the whole reef- 

 ground per year would not exceed one-eighth of an 

 inch. 



" Now some reefs are at least two thousand feet thick, 

 which at one-eighth of an inch a year, corresponds 

 to one hundred and ninety-two thousand years." l 

 1 Dana, Manual of Geology, P- 591. 



