MODE OF FORMATION. POT 
3. RATE OF GROWTH OF REEFS. 
The formation of a reef has been shown to be a very different pro- 
cess 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 opportunity for definite conclu- 
sions. . 
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. 
6. 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 may be 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 neighbouring 
plantations. 
c. It is also necessary to examine into whatever has any bearing 
upon the marine or tidal currents of the region—their strength, velo- 
city, direction, where they eddy, and where not, whether they flow 
over reefs that may afford debris or not. All the debris of one planta- 
tion may sometimes be swept away by currents to contribute to other 
patches, so that one will enlarge at the expense 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 ex- 
tend more rapidly than inner reefs. The former have the full action 
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