FORMATION OF CORAL REEFS AND ISLANDS. Oi 
Oo 
Il. RATE OF GROWTH OF REEFS. 
The formation of a reef has been shown to be a very dif- 
ferent process from the growth of a zodphyte. 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 zodphytes 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. 
b. The character of the coral plantation under consider- 
ation should be carefully studied; for it is of the greatest con- 
sequence to know whether the clusters of zodphytes are scat- 
tered tufts over a barren plain, or whether in crowded profu- 
sion. Compare the débris of vegetation on the semi-deserts of 
California with that of regions buried in foliage; equally va- 
rious 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 neighboring 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 débris or not. All the débris of one plantation may 
sometimes be swept away by currents to contribute to other 
patches, so that one will enlarge at the expense of others. Or, 
