vi.] ON CORAL AND CORAL REEFS. 131 



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 madrepore 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 



eoral 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. 



1 Shells and other organic relics may contribute one-fourth as much as 



; corals. At the outside, the average upward increase of the whole 



i 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." * 



Halve, or quarter, this estimate if you will, in order 

 i to be certain of erring upon the right side, and still there 

 remains a prodigious period during which the ancestors 

 1 of the existing coral polypes have been undisturbedly at 

 work ; and during which, therefore, the climatal condi- 

 tions over the coral area must have been much what 

 they are now. 



And all this lapse of time has occurred within the 

 most recent period of the history of the earth. The 



1 Dana, " Manual of Geology/ 7 p. 591. 

 7 



