212 CORALS AND CORAL ISLANDS. 



coves of land to give force to the waters of currents, and to 

 direct their course, and the absence also of fresh-water streams, 

 are the only modifying causes not present. It is readily under- 

 stood, therefore, why lagoon entrances are more likely to become 

 filled up by growing coral, than the passages through barrier 

 reefs. 



HI. RATE OF GROWTH OF REEFS. 



The formation of a reef has been shown to be a very 

 different process from the growth of a zoophyte. Its rate of 

 progress is a question to be settled by a consideration of 

 many distinct causes, none of which have yet been properly 

 measured. 



a. The rapidity of the growth of zoophytes is an element 

 in this question of great importance, and one that should be 

 determined by direct observation with respect to each of the 

 species which contribute largely to reefs, both in the warmer 

 and colder parts of coral-reef seas. 



/;. The character of the coral plantation under consideration 

 should be carefully studied ; for it is of the greatest conse- 

 quence to know whether the clusters of zoophytes are scattered 

 tufts over a barren plain, or whether in crowded profusion. 

 Compare the debris of vegetation on the semi-deserts of 

 California with that of regions buried in foliage ; equally 

 various may be the rate of growth of coral rock in different 

 places. An allowance should also be made for the shells and 

 other reef relics. The amount of reef-rock formed in a given 

 time cannot exceed, in cubic feet, the aggregate of corals and 

 shells added by growth — that is, if there are no additions from 

 other distant or neighbouring plantations. 



c. It is also necessary to examine all conditions that are 

 connected with, or can influence, the marine or tidal currents 

 of the region — their strength, velocity, direction, where they 

 eddy, and where not, whether they flow over reefs that may 

 afford debris or not. All the debris of one plantation may 

 sometimes be swept away by currents to contribute to other 



