RATE OF GROWTH OF CORAL REEFS. 



213 



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 planta- 

 tion 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 v/hich may afl"ect the latter. 



No results with reference to this question of the rate of 

 ])rogress 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, mentioned 

 on page 99, 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. cervicortiis 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 

 part annually added. This ascertained, it would be easy to 

 calculate how much the added coral would, if ground up, raise 

 the area that is covered by the Madrepora. A rough esti- 

 mate gives the author an average increase to this surface of 

 a fourth of an inch a year. But this fourth must be much 



