RATE OF GROWTH OF REEFS. 327 



In conclusion, I may at once remark that the above table absolutely negatives my 

 previously-expressed view' that the reef corals probably live down to 40 — 50 fethoms in 

 great luxuriance. Darwin in placing the extreme depth of flourishing banks of surface-reef 

 corals at 25 fathoms was entirely correct. The specimens of these corals procured from beyond 

 this depth show clearly in their growth that the increased depth is deleterious to them. At 

 the same time the presence of a series of other genera of corals, which evidently flourish 

 iust beyond where the surface forms cease to exist, although with a considerable range in 

 depth, is for the fii'st time clearly demonstrated. 



Section IV. The Rate of Growth of Corals and Reefs. 



The formation of a reef is an exceedingly complex problem, in which a large variety 

 of organisms all play their parts. The rate of growth of many reefs is doubtless infinitesimal. 

 Our scientific interest lies mainly in considering the probable rate of growth on open banks 

 freely exposed to the great ocean currents, while the practical interests of navigation require 

 as well that the rate of growth within the lagoons of atolls should be accurately ascertained. 

 Our practical interests demand the consideration of the possible, maximum rate of growth, 

 while for the theory we require rather the knowledge of the probable, average rates of growth. 



Freely exposed ocean banks and lagoons differ so profoundly in the structure of those 

 reefs that have grown up actually on or in them, that any comparison is at first sight 

 impossible. The lagoon reefs are more or less spongy structures, formed mainly of corals 

 filled in with sand, while to build up formations outside an atoll requires a far greater 

 complexity of organisms. The possible maximum rate of growth of a lagoon shoal is obviously 

 the greatest rate of growth of corals in heiglit in that position, the sand, etc., by which 

 the corals are consolidated, not being necessarily formed by the decay of its organisms. 

 These same shoals consist in the first place, until they reach the low tide limit, of mere 

 coral heads, and their probable rate of growth up to this stage is not unlikely to be nearly 

 equal to their possible rate. In addition the possible rate of lateral growth of such shoals 

 is required for practical purposes. The tendency to and possibility of such kind of growth 

 has already been sufficiently discussed in previous chapters, and need not detain us here. 

 It is, peradventure, sufficient to point out that for navigation the accurate fixation of the 

 central point of an ordinary shoal is all that is usually required with a rough contour and, 

 of course, the soundings. 



The rate of growth outside atolls is another matter. The general contour of the bottom 

 is smooth, though hollows and pits occur. All its corals grow at about the same rate and 

 so preserve its general contour. This doubtless is due to the strong outward cuiTcnt from 

 the reef above, and is absolutely comparable to the level top of a plantation of firs, on 

 a smooth, exposed hill-side. Rarely does any coral succeed in raising its head above the 

 general level, and, when one does so, it is improbable that it is able to remain as a permanent 

 structure. The loose material, that serves to fill in the interspace between the coral masses, 

 consists mostly of coral fragments — derived principally fi-om the reef flat above — and of free- 

 living Foraminifera. Lithothamnion and Polytrema turn the whole into rock, loss being 

 almost an unknown factor here. The rate of growth of the whole reef is, hence, not probably 

 widely different from that of the individual corals that mainly serve to build it up. Once 



1 Loc. cit. p. 478, etc. 

 G. 42 



