250 CORALS AND CORAL ISLANDS. 



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 zoophytes, 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 that other Madrepores increase in the same ratio, 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 M. 

 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 



