RATE OF GROWTH OF REEFS. 85 



3. Rate of Growth of Reefs. 



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

 ent process from the growth of a zoophyte. Its rate of progress 

 is a question to be settled by a consideration of many distinct 

 causes, and the rapid voyage of an Expedition affords no oppor- 

 tunity for definite conclusions. 



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

 this question of great importance, and one that should be deter- 

 mined 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. 



b. The character of the coral plantation under consideration 

 should be carefully studied : for it is of no little consequence 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 semideserts of California with that of 

 regions buried in foliage; equally various maybe the rate of 

 growth of coral rock in different places. Some 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 neighboring planta- 

 tions. 



c. It is also necessary to examine into whatever has any bear- 

 ing upon 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 de- 

 bris of one plantation may sometimes be swept away by currents 

 to contribute to other patches, so that one will enlarge at the ex- 

 pense of others. Or, currents may carry the detritus into the 

 channels or deeper waters around a coral patch, and leave little 

 to aid the plantation itself in its increase and consolidation. 



d. The course and extent of fresh waters from the land, and 

 their detritus, should be ascertained. 



e. The strength and height of the tides, and general force of 

 the ocean waves, will have some influence. 



Owing to the action of these causes, barrier reefs enlarge and 

 extend more rapidly than inner reefs. The former have the full 

 action of the sea, and are farther removed from the deleterious 

 influences which may affect the latter. 



As stated above, no results were arrived at from observations 

 made in the course of the voyage through the Pacific. The gen- 

 eral impression that their progress is slow, was fully sustained. 

 The facts, with regard to the growth of zoophytes, give some 

 data, though by no means satisfactory. 



