954 CORALS AND CORAL ISLANDS. 
a 
currents may carry the detritus into the channels or deeper 
waters around a coral patch, and leave little to aid the plan- 
tation 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 to aid them, and are farther removed 
from the deleterious influences which may affect the latter. 
No results with reference to this question of the rate of 
progress in reefs were arrived at by the author in the course 
of his observations in the Pacific. The general opinion, — 
that their progress is exceedingly slow, was fully sustained. 
The facts with regard to the growth of zodphytes, give some 
data. 
Allowing that the large Madrepora of the wreck, men- 
tioned on page 126, may grow three inches in height a year, 
and other Madrepores probably three to four inches, it is still 
not easy to deduce from it the rate of increase of the reef: In 
the first place, the whole Madrepore is growing over the sides 
of its branches, at the rate, if we may judge from the size of the 
trunk at base, of a tenth of an inch a year, thus increasing 
annually the diameter a fifth of an inch a year, which, in a 
large species, is a very great addition to the three inches per 
year at the extremities of the branches. Again, the branches of 
the large Madrepore of the wreck were widely spaced, those of JZ, 
cervicornis, having intervals of from six to eighteen inches or 
more between the branches. 
In fact it is impossible to make any exact estimate of the 
amount of increase without a knowledge of the weight of the 
